When all units started compiling, but not all are finished yet, the
main thread waits in a busy loop.
This CL fixes that by introducing a semaphore which is signalled when
baseline compilation finishes or compilation fails. The foreground
thread waits on this semaphore if there are no more units to start.

R=mstarzinger@chromium.org

Bug: v8:8916
Change-Id: I7351c0b777f008fef3aa1d1d16089c4e6fd91106
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564055Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60821}