BigInts are immutable after initialization, thus an acquire-release
synchronization point is sufficient to read safely from the background
thread. This CL introduces the `length` field as that sync point.

Bug: v8:7790
Change-Id: I977f30836b311c3851896dd29e708f78a090f547
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2854745
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74245}