Memory64 currently does not use trap handling, so we should not allocate
a guard region (10GB total reservation).
This is implemented by adding a {WasmMemoryFlag} enum in the backing
store header, which replaces the previous {MemoryIndexType}. The flag is
not stored with the backing store, as the backing store does not care
about the index type, and we might want to share the same backing store
for memory32 and memory64 (if sizes permit this).
Instead, we (still) store the flag with the WasmMemoryObject and pass it
to the backing store methods.

R=jkummerow@chromium.org

Bug: v8:10949
Change-Id: I284b85b98d181ba5e8d454b24bfa48f6ac201be5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3789506Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82038}