[Bug c++/91719] gcc compiles seq_cst store on x86-64 differently from clang/icc

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91719

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Surely this is a compiler bug not libstdc++ bug, since std::atomic just uses
the __atomic built-in function.