Add ARMv8-A to the supporting list of arm platforms

BUG=chrome-os-partner:36669
R=jochen@chromium.org
LOG=N
Signed-off-by: Milton Chiang <milton.chiange@mediatek.com>

Review URL: https://codereview.chromium.org/1223613003

Cr-Commit-Position: refs/heads/master@{#31691}

AUTHORS

@@ -82,6 +82,7 @@ Michael Lutz <michi@icosahedron.de>
 Michael Smith <mike@w3.org>
 Mike Gilbert <floppymaster@gmail.com>
 Mike Pennisi <mike@mikepennisi.com>
+Milton Chiang <milton.chiang@mediatek.com>
 Nicolas Antonius Ernst Leopold Maria Kaiser <nikai@nikai.net>
 Paolo Giarrusso <p.giarrusso@gmail.com>
 Patrick Gansterer <paroga@paroga.com>

src/base/atomicops_internals_arm_gcc.h

@@ -59,7 +59,8 @@ inline void MemoryBarrier() {
 // variant of the target architecture is being used. This tests against
 // any known ARMv6 or ARMv7 variant, where it is possible to directly
 // use ldrex/strex instructions to implement fast atomic operations.
-#if defined(__ARM_ARCH_7__) || defined(__ARM_ARCH_7A__) ||  \
+#if defined(__ARM_ARCH_8A__) || \
+    defined(__ARM_ARCH_7__) || defined(__ARM_ARCH_7A__) ||  \
     defined(__ARM_ARCH_7R__) || defined(__ARM_ARCH_7M__) || \
     defined(__ARM_ARCH_6__) || defined(__ARM_ARCH_6J__) ||  \
     defined(__ARM_ARCH_6K__) || defined(__ARM_ARCH_6Z__) || \