We add a checked build mode that asserts when mono mallocs inside of
the crash reporter. It makes risky allocations into assertions. It's
useful for automated testing because the double-abort often represents
itself as an indefinite hang. If it happens before the thread dumping
supervisor process is started, or after it ends, the crash reporter
hangs.

Threads without domains that get segfaults will end up in
this handler. It's not safe to call this function with a NULL domain.

See crash below:

```
* thread mono#1, name = 'tid_307', queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x10eff40f8)
  * frame #0: 0x000000010e1510d9 mono-sgen`mono_threads_summarize_execute(ctx=0x0000000000000000, out=0x0000001000000000, hashes=0x0000100000100000, silent=4096, mem="", provided_size=2199023296512) at threads.c:6414
    frame mono#1: 0x000000010e152092 mono-sgen`mono_threads_summarize(ctx=0x000000010effda00, out=0x000000010effdba0, hashes=0x000000010effdb90, silent=0, signal_handler_controller=1, mem=0x0000000000000000, provided_size=0) at threads.c:6508
    frame mono#2: 0x000000010df7c69f mono-sgen`dump_native_stacktrace(signal="SIGSEGV", ctx=0x000000010effef48) at mini-posix.c:1026
    frame mono#3: 0x000000010df7c37f mono-sgen`mono_dump_native_crash_info(signal="SIGSEGV", ctx=0x000000010effef48, info=0x000000010effeee0) at mini-posix.c:1147
    frame mono#4: 0x000000010de720a9 mono-sgen`mono_handle_native_crash(signal="SIGSEGV", ctx=0x000000010effef48, info=0x000000010effeee0) at mini-exceptions.c:3227
    frame mono#5: 0x000000010dd6ac0d mono-sgen`mono_sigsegv_signal_handler_debug(_dummy=11, _info=0x000000010effeee0, context=0x000000010effef48, debug_fault_addr=0xffffffffffffffff) at mini-runtime.c:3574
    frame mono#6: 0x000000010dd6a8d3 mono-sgen`mono_sigsegv_signal_handler(_dummy=11, _info=0x000000010effeee0, context=0x000000010effef48) at mini-runtime.c:3612
    frame mono#7: 0x00007fff73dbdf5a libsystem_platform.dylib`_sigtramp + 26
    frame mono#8: 0x0000000110bb81c1
    frame mono#9: 0x000000011085ffe1
    frame mono#10: 0x000000010dd6d4f3 mono-sgen`mono_jit_runtime_invoke(method=0x00007faae4f01fe8, obj=0x0000000000000000, params=0x00007ffee1eaa180, exc=0x00007ffee1ea9f08, error=0x00007ffee1eaa250) at mini-runtime.c:3215
    frame mono#11: 0x000000010e11509d mono-sgen`do_runtime_invoke(method=0x00007faae4f01fe8, obj=0x0000000000000000, params=0x00007ffee1eaa180, exc=0x0000000000000000, error=0x00007ffee1eaa250) at object.c:2977
    frame mono#12: 0x000000010e10d961 mono-sgen`mono_runtime_invoke_checked(method=0x00007faae4f01fe8, obj=0x0000000000000000, params=0x00007ffee1eaa180, error=0x00007ffee1eaa250) at object.c:3145
    frame mono#13: 0x000000010e11aa58 mono-sgen`do_exec_main_checked(method=0x00007faae4f01fe8, args=0x000000010f0003e8, error=0x00007ffee1eaa250) at object.c:5042
    frame mono#14: 0x000000010e118803 mono-sgen`mono_runtime_exec_main_checked(method=0x00007faae4f01fe8, args=0x000000010f0003e8, error=0x00007ffee1eaa250) at object.c:5138
    frame mono#15: 0x000000010e118856 mono-sgen`mono_runtime_run_main_checked(method=0x00007faae4f01fe8, argc=2, argv=0x00007ffee1eaa760, error=0x00007ffee1eaa250) at object.c:4599
    frame mono#16: 0x000000010de1db2f mono-sgen`mono_jit_exec_internal(domain=0x00007faae4f00860, assembly=0x00007faae4c02ab0, argc=2, argv=0x00007ffee1eaa760) at driver.c:1298
    frame mono#17: 0x000000010de1d95d mono-sgen`mono_jit_exec(domain=0x00007faae4f00860, assembly=0x00007faae4c02ab0, argc=2, argv=0x00007ffee1eaa760) at driver.c:1257
    frame mono#18: 0x000000010de2257f mono-sgen`main_thread_handler(user_data=0x00007ffee1eaa6a0) at driver.c:1375
    frame mono#19: 0x000000010de20852 mono-sgen`mono_main(argc=3, argv=0x00007ffee1eaa758) at driver.c:2551
    frame mono#20: 0x000000010dd56d7e mono-sgen`mono_main_with_options(argc=3, argv=0x00007ffee1eaa758) at main.c:50
    frame mono#21: 0x000000010dd5638d mono-sgen`main(argc=3, argv=0x00007ffee1eaa758) at main.c:406
    frame mono#22: 0x00007fff73aaf015 libdyld.dylib`start + 1
    frame mono#23: 0x00007fff73aaf015 libdyld.dylib`start + 1
  thread mono#2, name = 'SGen worker'
    frame #0: 0x000000010e2afd77 mono-sgen`mono_get_hazardous_pointer(pp=0x0000000000000178, hp=0x000000010ef87618, hazard_index=0) at hazard-pointer.c:208
    frame mono#1: 0x000000010e0b28e1 mono-sgen`mono_jit_info_table_find_internal(domain=0x0000000000000000, addr=0x00007fff73bffa16, try_aot=1, allow_trampolines=1) at jit-info.c:304
    frame mono#2: 0x000000010dd6aa5f mono-sgen`mono_sigsegv_signal_handler_debug(_dummy=11, _info=0x000070000fb81c58, context=0x000070000fb81cc0, debug_fault_addr=0x000000010e28fb20) at mini-runtime.c:3540
    frame mono#3: 0x000000010dd6a8d3 mono-sgen`mono_sigsegv_signal_handler(_dummy=11, _info=0x000070000fb81c58, context=0x000070000fb81cc0) at mini-runtime.c:3612
    frame mono#4: 0x00007fff73dbdf5a libsystem_platform.dylib`_sigtramp + 26
    frame mono#5: 0x00007fff73bffa17 libsystem_kernel.dylib`__psynch_cvwait + 11
    frame mono#6: 0x00007fff73dc8589 libsystem_pthread.dylib`_pthread_cond_wait + 732
    frame mono#7: 0x000000010e28d76d mono-sgen`mono_os_cond_wait(cond=0x000000010e44c9d8, mutex=0x000000010e44c998) at mono-os-mutex.h:168
    frame mono#8: 0x000000010e28df4f mono-sgen`get_work(worker_index=0, work_context=0x000070000fb81ee0, do_idle=0x000070000fb81ed4, job=0x000070000fb81ec8) at sgen-thread-pool.c:165
    frame mono#9: 0x000000010e28d2cb mono-sgen`thread_func(data=0x0000000000000000) at sgen-thread-pool.c:196
    frame mono#10: 0x00007fff73dc7661 libsystem_pthread.dylib`_pthread_body + 340
    frame mono#11: 0x00007fff73dc750d libsystem_pthread.dylib`_pthread_start + 377
    frame mono#12: 0x00007fff73dc6bf9 libsystem_pthread.dylib`thread_start + 13
  thread mono#3, name = 'Finalizer'
    frame #0: 0x00007fff73bf6246 libsystem_kernel.dylib`semaphore_wait_trap + 10
    frame mono#1: 0x000000010e1d9c0a mono-sgen`mono_os_sem_wait(sem=0x000000010e43e400, flags=MONO_SEM_FLAGS_ALERTABLE) at mono-os-semaphore.h:84
    frame mono#2: 0x000000010e1d832d mono-sgen`mono_coop_sem_wait(sem=0x000000010e43e400, flags=MONO_SEM_FLAGS_ALERTABLE) at mono-coop-semaphore.h:41
    frame mono#3: 0x000000010e1da787 mono-sgen`finalizer_thread(unused=0x0000000000000000) at gc.c:920
    frame mono#4: 0x000000010e152919 mono-sgen`start_wrapper_internal(start_info=0x0000000000000000, stack_ptr=0x000070000fd85000) at threads.c:1178
    frame mono#5: 0x000000010e1525b6 mono-sgen`start_wrapper(data=0x00007faae4f31bd0) at threads.c:1238
    frame mono#6: 0x00007fff73dc7661 libsystem_pthread.dylib`_pthread_body + 340
    frame mono#7: 0x00007fff73dc750d libsystem_pthread.dylib`_pthread_start + 377
    frame mono#8: 0x00007fff73dc6bf9 libsystem_pthread.dylib`thread_start + 13
  thread mono#4
    frame #0: 0x00007fff73c0028a libsystem_kernel.dylib`__workq_kernreturn + 10
    frame mono#1: 0x00007fff73dc7009 libsystem_pthread.dylib`_pthread_wqthread + 1035
    frame mono#2: 0x00007fff73dc6be9 libsystem_pthread.dylib`start_wqthread + 13
(lldb)
```

Each frame that prints ends up increased by the size of buff.
In practice, clang often fails to deduplicate some of these buffers,
leading to 30k-big stackframes.

It was noticed by a series of hard-to-diagnose segfaults on stacks that
looked otherwise fine during the crash reporting stress test.

This change fixes this, making stacks a 1/10th of the size. It doesn't
seem to break the crash reporter messages anywhere (may need to shrink
other "max name length" fields), and it's not mission-critical anywhere
else.