The n5x bot fails to *allocate* more memory just before we reach the
soft limit of 512 GiB for *reserved* memory. Thus reduce it a bit. This
still leaves enough room for ~38 wasm memories with full guard regions.

R=ahaas@chromium.org

Bug: v8:8196
Change-Id: I6049e2baa3fc8b99c9269f8cb7c013a6e77ebf9b
Reviewed-on: https://chromium-review.googlesource.com/1235924Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56084}