The VMAR syscall signatures are being updated, so migrate to the
temporarily-provided *_old() variants, in advance of the Fuchsia SDK
roll that updates the real APIs.

TBR: hpayer
Change-Id: I10da4df97b122262de04a2417bd91b921f3a63d5
Reviewed-on: https://chromium-review.googlesource.com/1194429
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55473}