This patch to the GCC-specific part of the Go frontend defines the
__atomic_fetch_add functions.  The frontend only generates calls to
the __atomic_add_fetch functions, but in some cases the middle-end can
transform the latter into the former.  This patch is originally by
Marc Poulhiès.  This fixes PR  107581.  Bootstrapped and ran Go
testsuite on x86_64-pc-linux-gnu.  Committed to mainline.

Ian