[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 #7 from Uroš Bizjak <ubizjak at gmail dot com> ---
Actually, x86_64 (at least my Fedora 32) does not like operations on stack:

Starting program: /sdd/uros/git/gcc/gcc/testsuite/gcc.dg/atomic/a.out 

Program received signal SIGSEGV, Segmentation fault.
0x000000000040110a in main ()
(gdb) disass
Dump of assembler code for function main:
   0x0000000000401106 <+0>:     push   %rbp
   0x0000000000401107 <+1>:     mov    %rsp,%rbp
=> 0x000000000040110a <+4>:     lock orq $0x0,(%esp)
   0x0000000000401111 <+11>:    mov    $0x0,%eax
   0x0000000000401116 <+16>:    pop    %rbp
   0x0000000000401117 <+17>:    retq   
End of assembler dump.

I didn't investigate further, but 32bit executable works OK.