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

[andysem at mail dot ru]

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

--- Comment #2 from andysem at mail dot ru ---
gcc 10.1 only optimizes __atomic_store_n(seq_cst) to xchg, but not the fence.

Also, consider applying the same optimization to __sync_synchronize().