depot_tools: Use armv6 for cipd's GOARCH when on armv7 host.

We only compile and tag cipd packages with armv6, so armv7 hosts get
404s from cipd. This will use armv6 packages on armv7 bots, which should
work.

Bug: 776703
Change-Id: I6e54c2f9e1b220ce038a99af1df28cb6c0a9e579
Reviewed-on: https://chromium-review.googlesource.com/730850Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>

cipd

@@ -44,6 +44,9 @@ case $UNAME in
   aarch64)
     ARCH=arm64
     ;;
+  armv7l)
+    ARCH=armv6l
+    ;;
   arm*)
     ARCH=$UNAME
     ;;