This extends the jump table stress test. Currently, we generate
different thunks (on the main thread) and then concurrently update the
jump table to jump to one of these thunks.
With this CL, we also generate the thunks concurrently. So this also
tests whether there is proper synchronization between code generation
and executing it in another thread.

R=ahaas@chromium.org, mstarzinger@chromium.org

Bug: v8:9477
Change-Id: I3598329e37482ebd27a13acc752581c714226184
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1735319Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63097}