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

[redi at gcc dot gnu.org]

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

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
libatomic provides a definition for __atomic_compare_exchange_16 when the
native instruction isn't available.