Avoid name clash with Safepoint in src/codgen and use renaming to
emphasize that class reaches a safepoint in each background thread.

Bug: v8:10315
Change-Id: I391cdcfaf533a0fe0d5923207234eb2a8411eb93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2139576Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67037}