compiler: intrinsify runtime/internal/atomic functions

compiler: intrinsify runtime/internal/atomic functions

    Currently runtime/internal/atomic functions are implemented in C
    using C compiler intrinsics. This CL lets the Go frontend
    recognize these functions and turn them into intrinsics directly.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176918

	* go-gcc.cc (Gcc_backend::Gcc_backend): Define atomic builtins.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271308 138bc75d-0d04-0410-961f-82ee72b054a4