When sandboxed external pointers are enabled, external pointers now only
require 32 bits of storage space in a HeapObject. This CL does not shrink
the size of EmbedderDataSlots, which will happen in a follow-up CL.

Bug: v8:10391
Change-Id: I3cf8b68c3b985cf806a45183717f50462a88c281
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3359629Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78754}