IsolateData guarantees a fixed root-relative offset for its contents,
thus allowing more efficient code generation for accesses to these
addresses. The stack limit, located within the StackGuard, is used by
all stack checks in CSA.

This CL moves the StackGuard inside IsolateData to make such efficient
loads of the limit possible.

Bug: v8:9595,v8:9534
Change-Id: I9abe26b88952709c88bf625cc6c028497815a58c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1741648Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63160}