This patch maintains the previous default value of the flag controlling
the max size of Wasm memories, but allows the limit to be raised on the
command line.
Bonus content: improve the multi-mapped mock allocator by falling back
to regular allocation for small requests.
More bonus content: make debug-mode Wasm tests faster.

Bug: v8:6306
Change-Id: Idabae5734794b06e65d45b3a6165dbd488847f3f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1981157
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65681}