[Bug target/80878] -mcx16 (enable 128 bit CAS) on x86_64 seems not to work on 7.1.0

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878

--- Comment #20 from Avi Kivity <a...@cloudius-systems.com> ---
Note that clang generates cmpxchg16b when the conditions are ripe:

https://godbolt.org/z/j9Whgh