[Bug target/95750] [x86] Use dummy atomic insn instead of mfence in __atomic_thread_fence(seq_cst)

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95750

--- Comment #10 from Uroš Bizjak <ubizjak at gmail dot com> ---
Created attachment 48756
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48756&action=edit
Proposed patch

Patch in testing, survives GOMP testcases.

On a related note, the patch uses TARGET_USE_XCHG_FOR_ATOMIC_STORE, which
should probably be renamed to something more appropriate.