[Bug target/87231] stdatomic.h: No such file or directory on OpenBSD 6.3

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87231

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Similar issue to PR 69744.  OpenBSD config should just remove that.