This CL implements most parts of the concurrent maglev pipeline.

- MaglevConcurrentDispatcher: controls concurrent jobs.
- MaglevCompilationInfo: holds job-global data, controls handle
  fiddling between the main isolate and local isolates, owns
  job-global state like the Zone.
- MaglevCompilationUnit: same as before, holds per-unit data.

Still missing: job finalization.

Bug: v8:7700
Change-Id: I281178d945e79a0ba97fa2ac7023285d84a16641
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3516036Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79489}