Add the object start bit when adding an entry to the free list.
Introduce a GC at the end of the tests to make sure that the object
start bitmap verification catches such issues.

Bug: chromium:1056170
Change-Id: Id8ca257ce054fc3fb199955cf1c4f38004033747
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2752870Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73353}