Limit the number of platforms that this is done on.  A inspection of
some libc implementations of fork has identified the main culprits,
don't need to apply this to any others.

MacOS testsuite also regressed as a result on calling this code, it's
not clear why, but the backtrace is:
```
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
  * frame #0: 0x00007ff81abe6ee3 libsystem_platform.dylib`_os_unfair_lock_recursive_abort + 23
    frame #1: 0x00007ff81abe12da libsystem_platform.dylib`_os_unfair_lock_lock_slow + 247
    frame #2: 0x00007ff81abccd44 libsystem_pthread.dylib`_pthread_atfork_prepare_handlers + 48
    frame dlang#3: 0x00007ff825dc2705 libSystem.B.dylib`libSystem_atfork_prepare + 25
    frame dlang#4: 0x00007ff81aac17e1 libsystem_c.dylib`fork + 24
    frame dlang#5: 0x0000000101f730ee test_runner`core.internal.backtrace.dwarf.resolveAddressesWithAtos(Location[]) + 210
```