weak dependencies with -trans-mod (github/ocamllabs/weak-depends 45e980a,21856a7,merge)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14719 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
initialize blocks with Val_unit, not zero

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14720 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02