AllocatePages (used to allocate the embedded blob's backing store
during mksnapshot) has allocation address, size, and alignment
parameters. Both address and size are expected to be aligned, but we
were only aligning size properly. This CL also aligns the address (and
adds a bunch of comments as well).

Bug: v8:9677
Change-Id: Ia739682236c74278bcaf1c9b7c9c4b3e0b0c5582
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1784277
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63538}