People change their names and email addresses for many reasons. Adding
a `.mailmap` ensures that e.g. `git log` and `git blame` respect
people’s choices.

Bug: chromium:1213438
Change-Id: I03fcacff90d996d423283c345ddfc4ed9fccf98e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2919671Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74799}