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

[amonakov at gcc dot gnu.org]

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

--- Comment #8 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
Well, at least it's not too late to update the compiler manual, so I've
submitted a patch: https://gcc.gnu.org/ml/gcc-patches/2017-05/msg02080.html