Instead of potentially mapping a larger region than requested, then
unmapping parts of it to obtain a mapping with the desired alignment, we
now compute the ZX_VM_ALIGN_X constant corresponding to the specified
alignment and use that in the call to zx_vmar_map.

Bug: chromium:1218005
Change-Id: Ia36de6a06f3f2d625d177320d7e46fd29331f711
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3307054Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78164}