[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

Uroš Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #48756|0                           |1
        is obsolete|                            |
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ubizjak at gmail dot com
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-07-20

--- Comment #11 from Uroš Bizjak <ubizjak at gmail dot com> ---
Created attachment 48897
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48897&action=edit
Proposed patch

Patch in testing.