fix os2.process_exec on non-windows and add a smoke test

[laytan]

draft because ci is failing sporadically on this, need to look into that

[laytan]

Fixed a bunch of bugs here, FreeBSD and RISC-V Linux still misbehaving

[laytan]

Ok found and fixed the FreeBSD issue, the RISC-V Linux deadlock is left.

[laytan]

The QEMU we run in CI does not support execveat and that call is erroring with ENOSYS, then the code tries to reap the process here:

Odin/core/os/os2/process_linux.odin

Line 597 in dd7c889

_reap_terminated(process)

which launches this waitid:

Odin/core/os/os2/process_linux.odin

Line 661 in dd7c889

errno = linux.waitid(.PID, linux.Id(process.pid), &info, {.WEXITED}, nil)

and that does not return until the CI timeout of 15 minutes is up.

I do not know why that waitid never returns yet.