The stack memory will be accessed through a global list later, so the
stack pointer should be accessible without the containing
WasmContinuationObject. This also saves some unnecessary allocations and
indirections already.

R=ahaas@chromium.org

Bug: v8:12191
Change-Id: Ic3d71ecadbb13b18f0440049527ba71d657589b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3312486
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78215}