- Uses a mutex to prevent races on getting random mmap addresses, on
  POSIX and Windows.

Original change's description:
> [Memory] Speculative fix for sanitizer flakiness.
>
> - When allocating virtual memory, make sure addresses don't interfere
>   with hard-coded sanitizer regions.
>
> Bug: v8:7146
> Change-Id: I5bcb664b32bf53c8581772fe329190da6033701f
> Reviewed-on: https://chromium-review.googlesource.com/833171
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50208}

Bug: v8:7146
Change-Id: I5a82f2a1f6136498fb2aa7a37e0206c506545073
Reviewed-on: https://chromium-review.googlesource.com/834453Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50216}