MFC 356951: posix_spawn: mark error as volatile

In the case of an error, the RFSPAWN'd thread will write back to psa->error
with the correct exit code. Mark this as volatile as the return value is
being actively dorked up for erroneous exits on !x86.

This fixes the following tests, tested on aarch64 (only under qemu, at the
moment):

• posix_spawn/spawn_test:t_spawn_missing
• posix_spawn/spawn_test:t_spawn_nonexec
• posix_spawn/spawn_test:t_spawn_zero