This change writes a magic number into the root register,
and checks that we have the number still in at StackChecks.

Change-Id: Ib00d2a7ede651997ae9459cf4bdcc6be3952353b
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/1186402
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55353}