[buildtools] Pull arm64 GN binary on arm64 Macs

The x64 binary will work with Rosetta, but not requiring
Rosetta is nicer.

Change-Id: I34f9e5304022c33ffd3bac5ae881bd4d94681356
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3038529
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75808}

DEPS

@@ -110,7 +110,7 @@ deps = {
   'buildtools/mac': {
     'packages': [
       {
-        'package': 'gn/gn/mac-amd64',
+        'package': 'gn/gn/mac-${{arch}}',
         'version': Var('gn_version'),
       }
     ],