Harden a few builtins

Introducing BUILTIN_ASSERT, builtins' equivalent of RUNTIME_ASSERT.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/292173011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00