This CL fixes two issues:

1) When the specified vmar_offset was zero, the previous logic would
   incorrectly conclude that no target address was specified, and would
   potentially place the allocation elsewhere in memory, not at the
   desired address. This CL now passes both the target address and the
   VMAR base address to AllocateInternal, which can then correctly
   determine whether a target address was supplied.

2) When the root_vmar was used and a hint specified, the previous logic
   would incorrectly use nullptr as base address of the root_vmar, which
   appears to be incorrect. The new logic now obtains the actual base
   (apparently 2MB) through zx_object_get_info during initialization.

Bug: v8:10391
Change-Id: Ia8215440a790b4a2a0c8d33f623d3ecb6a731a97
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3398506Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78693}