Add mac_xcode_version to gclient_gn_args

Bug: chromium:1100266
Change-Id: Iaa847a50e9a79261fe9d9050db5046fa99352eaa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2273122
Auto-Submit: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68580}

DEPS

@@ -4,6 +4,7 @@
 
 gclient_gn_args_file = 'v8/build/config/gclient_args.gni'
 gclient_gn_args = [
+  'mac_xcode_version',
 ]
 
 vars = {