OCaml bindings: implement Llvm_target_machine.

Conflicts:
	bindings/ocaml/Makefile
OCaml bindings: fix ABI incompatibility.

OCaml's type unit is not compatible with C's type void.
OCaml bindings: fix typo in documentation.
OCaml bindings: implement Llvm.MemoryBuffer.{of_string,as_string}.