On Darwin/ARM, because libSystem doesn't provide functions for
__sync_fetch_and_add, and only clang can inline that function,
skip the test when building with GCC.

Change-Id: Id5e9d8f9bbe1e6bcb2f381f0f66cf68aa95277c7
Reviewed-on: https://go-review.googlesource.com/2125Reviewed-by: Ian Lance Taylor <iant@golang.org>