[Bug target/94649] 16-byte aligned atomic_compare_exchange doesn not generate cmpxcg16b on x86_64

[a...@cloudius-systems.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94649

--- Comment #2 from Avi Kivity <a...@cloudius-systems.com> ---
Maybe we can have a new flag -mcx16all that assumes that all code using 16-byte
CAS is compiled with the flag.