This could be related to a change to the default ISA spec in recent gcc versions. You might need to specify --with-isa-spec=2.2 when configuring the toolchain.

Here are some related references:
riscv-collab/riscv-gnu-toolchain#1052
https://five-embeddev.com/riscv-user-isa-manual/Priv-v1.12/csr.html
https://gcc.gnu.org/onlinedocs/gcc/RISC-V-Options.html

Thanks, that pointed me in the right direction. I discovered this issue: riscv-software-src/riscv-tests#368 that I make take up. In the meantime, I discovered that you all have this file that defines the needed testing macros without any unsupported instructions https://github.com/risc0/zirgen/blob/main/zirgen/circuit/rv32im/v1/test/riscv_test.h. I replaced https://github.com/riscv/riscv-test-env/blob/master/p/riscv_test.h by that file and then updated https://github.com/riscv/riscv-test-env/blob/master/p/link.ld to define __global_pointer as is done here https://github.com/risc0/zirgen/blob/main/bazel/toolchain/risc0/risc0.ld#L42 and with this I have reproduced your test of the add opcode.

Relating to this, do you have a tldr for the user elf/kernel elf distinction in the code? I have another source of ELF files that I can currently run through RISC0, but I need to use the from_kernel_elf function which has some signs of deprecation(?).