This implements sign extension for the arm32 port of Liftoff.

Bug: v8:6600
Change-Id: Ib9fb56835b92fa96af013fd3504395d24a27e10e
Reviewed-on: https://chromium-review.googlesource.com/c/1348429
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57779}