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.

From-SVN: r271308