mac: Pull arm64 gn binary on M1 hosts.

See https://chromium-review.googlesource.com/c/chromium/src/+/2602663
for a full explanation (WebRTC is just following Chromiium on this).

Bug: chromium:1103319
Change-Id: I2b08b259a43398297d21799696c226a96099fd41
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/199961
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#32893}
diff --git a/DEPS b/DEPS
index f4cab06..31fc610 100644
--- a/DEPS
+++ b/DEPS
@@ -46,7 +46,7 @@
   'src/buildtools/mac': {
     'packages': [
       {
-        'package': 'gn/gn/mac-amd64',
+        'package': 'gn/gn/mac-${{arch}}',
         'version': 'git_revision:0d67e272bdb8145f87d238bc0b2cb8bf80ccec90',
       }
     ],