Add flag FLAG_concurrent_allocation that when enabled protects all runtime allocations in the old space with a mutex. Prerequisite for concurrent allocation. Still disabled by default.

Bug: v8:10315
Change-Id: I37e59b3d51fe3aad112f0b556ea867cc0a2496e2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2140937
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67075}