[OCaml] Use $CAMLORIGIN, an rpath-$ORIGIN-like mechanism in OCaml.

As a result, installations of LLVM in non-standard locations
will not require passing custom -ccopt -L flags when building
the binary, nor absolute paths would be embedded in the cma/cmxa
files. Additionally, the executables will not require changes
to LD_LIBRARY_PATH, although CAML_LD_LIBRARY_PATH still
has to be set for ocamlc without -custom.

See http://caml.inria.fr/mantis/view.php?id=6642.
Note that the patch is approved, but not merged yet.
It will be released in 4.03 and likely 4.02.

llvm-svn: 225778