Allow cipd to interpret UNAME aarch64 as arm64 architecture

BUG=768318
R=phajdan.jr@chromium.org
R=dnj@chromium.org
R=iannucci@chromium.org

Change-Id: I2c85f9d30e74cfaebf2718bee075d6a21a6e0421
Reviewed-on: https://chromium-review.googlesource.com/708738Reviewed-by: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: William Hesse <whesse@google.com>

cipd

@@ -35,6 +35,9 @@ case $UNAME in
   s390x)  # best-effort support for IBM s390x: crbug.com/764087
     ARCH=s390x
     ;;
+  aarch64)
+    ARCH=arm64
+    ;;
   arm*)
     ARCH=$UNAME
     ;;