Partially reland Auto-generate v8 version based on tags.

Partially reland Auto-generate v8 version based on tags.

This relands parts of
https://codereview.chromium.org/843913009

It prepares for using this script outside of v8, e.g. in a
chromium hook.

The script is intended to run as a hook and will create
version_gen.cc if the content has changed.

Changes to gyp and gn files can land as a follow up, once
calling the hook on the chromium side has landed.

BUG=chromium:446166
LOG=n

Review URL: https://codereview.chromium.org/830093003

Cr-Commit-Position: refs/heads/master@{#26144}