This implements the last bits of basic concurrent Maglev compilation.
When jobs have been processed, schedule an interrupt to trigger codegen
and building the Code object on the main thread.

Changed since the initial version:
- Put the include behind V8_ENABLE_MAGLEV.
- Skip 18.js until we have deterministic test helpers for concurrent
  tiering.

Bug: v8:7700
Change-Id: Ibc103f097fe00f7df93a33a785939e43901f3734
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3536662Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79539}