util: Add aarch64 atomic ops.

This adds necessary atomic ops for aarch64
so that RE2 builds.

LGTM=rsc
R=rsc
CC=re2-dev
https://codereview.appspot.com/69070043

CONTRIBUTORS

diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index afadc7a..2a694bc 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -27,6 +27,7 @@
 # Please keep the list sorted.
 
 Dominic Battré <battre@chromium.org>
+Doug Kwan <dougkwan@google.com>
 Dmitriy Vyukov <dvyukov@google.com>
 John Millikin <jmillikin@gmail.com>
 Mike Nazarewicz <mpn@google.com>