lang/v8: Fix fetch by fetching sub-dependencies from GitHub mirror

The +archive tarballs on chromium.googlesource.com are generated on
the fly with the time of the request as the mtime of every entry,
and the generated tarballs are not reproducible, so their size and
checksum change with each download and fetch fails with a size
or checksum mismatch.

To address this, fetch all sub-dependencies with GH_TUPLE from the
unofficial chromium-full-mirror organization on GitHub, which carries
the same commits and produces stable tarballs.

Because the chromium-full-mirror is an unofficial mirror, a new
verify-mirror target is added, which downloads every sub-dependency
from chromium.googlesource.com and compares the extracted tree with
the mirror distfile.

This new target is run as part of "make makesum" so distinfo can only
be generated when comparison passing.

Drop the hand written post-extract step as the framework now handles
the sub-dependencies.

verify-mirror passed for all 27 sub-dependencies. The patched source
tree is identical to the previous one except for the line endings of
two unused files in jsoncpp's devtools, so no PORTREVISION bump.