On OSes other than Windows and Fuchsia the write barrier assumes that
the caged heap is allocated below the stack.
Add CHECK that the assumption holds.

Bug: chromium:1056170
Change-Id: I64c790e61b4cfa2adb8274ed74111f0433e9aefb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3080570
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76173}