This patch to the Go frontend by Cherry Zhang intrinsifies the
runtime/internal/atomic functions.  Currently the
runtime/internal/atomic functions are implemented in C using C
compiler intrinsics.  This patch lets the Go frontend recognize these
functions and turn them into intrinsics directly.  Bootstrapped and
ran Go testsuite on x86_64-pc-linux-gnu.  Committed to mainline.

Ian

2019-05-16  Cherry Zhang  <cherryyz@google.com>

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