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

[fw at gcc dot gnu.org]

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

--- Comment #21 from Florian Weimer <fw at gcc dot gnu.org> ---
(In reply to Avi Kivity from comment #20)
> Note that clang generates cmpxchg16b when the conditions are ripe:
> 
> https://godbolt.org/z/j9Whgh

I believe this is a different, C++-specific issue. The C front end already
emits cmpxchgq in this situation.