The new platform can be used in combination with --single-threaded.
It disables background threads and thus avoids waiting on mutexes
and condition variables completely, which is useful for V8 embedders
that fork the V8 process after initialization.

As a bonus the new platform allows use to test --single-threaded and
has already uncovered an existing bug in parallel pointer updating code.

Change-Id: I3446fa027d2a077641cdaac0cd08062a1acae176
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2416501
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71187}