[Bug target/70821] x86_64: __atomic_fetch_add/sub() uses XADD rather than DECL in some cases

[dhowells at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70821

--- Comment #4 from dhowells at redhat dot com <dhowells at redhat dot com> ---
The patch works, thanks:

000000000000001c <test_atomic_dec_and_test>:
  1c:   f0 ff 0f                lock decl (%rdi)
  1f:   ba 00 00 00 00          mov    $0x0,%edx
  24:   b8 00 00 00 00          mov    $0x0,%eax
  29:   48 0f 44 c2             cmove  %rdx,%rax
  2d:   c3                      retq