Atomic ops on ARM are implemented with kernel calls, so they are
not intrinsified.

Change-Id: I0e7cc2e5526ae1a3d24b4b89be1bd13db071f8ef
Reviewed-on: https://go-review.googlesource.com/28977
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>