[Bug libstdc++/69744] OpenBSD does not define max_align_t

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69744

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
stddef.h is provided by GCC itself and thus likely needs "fixing" for OpenBSD
(see gcc/ginclude/stddef.h)