new -opaque flag: ocamlopt creates .cmx without cross-module info

(Patch by Pierre Chambart and Gabriel Scherer)
(Ack'ed by Damien Doligez)

This enables a form of incremental compilation for ocamlopt: if
A depends on B, and B is compiled with `ocamlopt -opaque`, then minor
implementation changes in B that do not affect the module interface
will not require recompilation of A.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15105 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02