Extend gen-postmortem-metadata.py with selected register values.

This information is not present in DWARF debuginfo. Exposing it
enables detailed analysis of V8 JS execution by observing binary-level
execution:
https://robert.ocallahan.org/2020/05/omniscient-js-debugging-in-pernosco.html

Bug: v8:11106
Change-Id: I3bde7dd07ac5ba6ff00d4a5fa9b635871507a866
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2518957
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71035}