Starboard is the platform abstraction for Cobalt.
This CL introduces all Cobalt changes in src/base/platform.

The review was conducted mostly on:
https://chromium-review.googlesource.com/c/v8/v8/+/2247918

See b/156155426 for background

Tbr: mlippautz@chromium.org
Change-Id: I6cd092304ba6485acd38e82aa2dc4505d7dfb0aa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2346090
Commit-Queue: John Xu <johnx@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69530}