We currently wait until baseline compilation is finished to finalize
export wrappers, but this can happen concurrently.
This change triggers export wrapper finalization as soon as the last
export wrapper is compiled, while background threads start compiling
baseline units.

R=clemensb@chromium.org

Bug: v8:9916
Change-Id: I945a8de7a917f6bc79e8a50431d8da530a901956
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2400342
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69884}