Use 'ocaml.' prefix for built-in attributes ocaml.warning, ocaml.deprecated.  Later, there will be a warning when an unknown ocaml.* attribute is used.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14591 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02