[OCaml] Embed rpath into stub libraries and native executables

This commit embeds a set of linker flags with hardcoded paths to
the LLVM shared library on --enable-shared builds into .cmxa files
and stub dynamic libraries. This solution closely follows existing
rules for rpath in the LLVM tools, which had to be modified because
of differences in toolchain.

Without this patch, OCaml tests as well as opam bindings broke,
as neither of those updates LD_LIBRARY_PATH to include
the $prefix/lib directory.

llvm-svn: 195834