Ensure that the jump tables do not take more than 50% of newly reserved
code spaces. In particular, this ensures that the jump tables always
fit in the newly reserved code space.

R=mstarzinger@chromium.org

Bug: v8:9477
Change-Id: I72cdbb7c7dc7916167594c0fc8e1ddb1511756ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1782559
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63687}