Tweak malloc_stack on OpenBSD to match other unices

[fingolfin]

The other malloc_stack implementations were switched to using jl_atomic_fetch_add_relaxed on 2024-02-23. I suspect that this simply overlapped in time with the creation of the OpenBSD patches, which were submitted in March, but presumably work started before that :-).

CC @semarie please check that I am not messing up something for you.

Also, while at it, I noticed that the OpenBSD malloc_stack is missing the mprotect for setting up a guard page, which concerns me a bit. Is there a deeper reason for this omission?