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

This reverts commit r16238 for not being able to use the defines
outside of libv8 (i.e. in tests). Will reland the #define's in a
build config header style.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/22861022

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16267 ce2b1a6d-e550-0410-aec6-3dcde31c8c00