[PR#6059, patch by Anil Madhavapeddy] add -output-obj rules and targets to ocamlbuild for byte and native code

Three corresponding targets are added:
%.byte.o   : bytecode object file
%.byte.c   : bytecode source C file
%.native.o : native code object file


git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13850 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02