The stack of an isolate's main thread is kept in the isolate's heap.
This CL sets the stack's start address when the isolate's heap is set
up; it can also be set explicitly from the embedder. The CL also fixes
threaded cctests, where an isolate is shared by many "main" threads.

Bug: v8:13257
Change-Id: Ie30bbbe4130882d94f23de946cbada748f32e22d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3870923Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82973}