Each GC thread needs their own instance of ConcurrentAllocator for
allocation. The LAB is always considered thread-local.

Bug: v8:12582, v8:11708
Change-Id: I39200202ec9fd07fa33b3ababa88e84a1a270778
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3429294Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78917}