[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 #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Since this makes libatomic required for DCAS on x86_64 it should probably have
been documented at https://gcc.gnu.org/gcc-7/changes.html