Only crosscompile binary op stubs if we compile a snapshot.

BUG=
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/26266005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17130 ce2b1a6d-e550-0410-aec6-3dcde31c8c00