In the process, switch to using the Fuchsia GN SDK templates for
building the component and package.

gni/v8.cmx is retained temporarily until out-of-tree consumers have been
updated.

Bug: v8:12589
Change-Id: If08cfcbf579696482e7cd60a8b8b80bcc4c7dab2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3885881
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Alexander Schulze <alexschulze@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83106}