Cleanup V8_TURBOFAN_TARGET definition.

Move to globals.h so we do no longer skip any number of
tests. Introduce V8_TURBOFAN_BACKEND, which is 1 if we
actually have a backend and want to compile it, but don't
run the tests.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/444503003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22863 ce2b1a6d-e550-0410-aec6-3dcde31c8c00