We only run one GC at a time. If another one should be triggered before
the previous one finished, that second one is just ignored.
This CL changes that to set a bit on the current GC to trigger another
one once it finished. This fixes situations where
a) a lot of code becomes dead after a GC is triggered but before it
   finished, but
b) no more code becomes dead after the GC.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: I5b5ca055124f467e1b0d67b8eb818069affc4a1d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598696Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61278}