Add a basic gn file for V8

Currently, the gn support is incomplete and only works from within a
chromium checkout.

See https://code.google.com/p/chromium/wiki/gn for details

We move the BUILD.gn file to v8 nevertheless to make renaming files
easier, as the chromium CQ already now checks the gn build.

BUG=none
R=brettw@chromium.org, machenbach@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/259233002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21140 ce2b1a6d-e550-0410-aec6-3dcde31c8c00