[google_sign_in] Mark iOS arm64 simulators as unsupported

[jmagman]

Before flutter/flutter#87244, any app using the google_sign_in plugin would not build on an M1 Mac.

Replace deprecated 'VALID_ARCHS[sdk=iphonesimulator*]' => 'x86_64' with 'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'arm64'. Once GoogleSignIn 6.0 is adopted, this build setting can be removed.

Fixes flutter/flutter#87243
See also #4209

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
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• I listed at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[jmagman]

Test failure to be fixed in flutter/flutter: flutter/flutter#87362

[stuartmorgan]

LGTM