Avoid name clashes of builtins and runtime functions.

This makes it possible to use %Percent() notation to call any given builtin or runtime function in tests.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/280243002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21298 ce2b1a6d-e550-0410-aec6-3dcde31c8c00