Add (mostly Chromium-compatible) V8_OS_* defines to v8.gyp.

R=jkummerow@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23283010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16238 ce2b1a6d-e550-0410-aec6-3dcde31c8c00