Use relative paths to access v8 root build rules since // means
something different in a chrome build.

Bug: v8:7330
Change-Id: I4eac73b394daaf0250b0fa2ad415af560b03c114
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2856840
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74258}