Rename functions for main thread allocation to indicate that they are
used for main thread allocation.

Bug: v8:10315
Change-Id: Idd359a7a439ec2e93f0bdc2f1bed987755790bbe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2292308Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68828}