Add a flag which causes wasm code gc to be triggered whenever any code
is found to be potentially dead. This mode found several bugs already,
and I plan to enable it in 'gc-stress' mode once all issues are fixed.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: If28d980ded98b77b9efe7446da74d857e3c5e1b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1585720
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61039}