[OCaml] Avoid embedding absolute paths into executables.

Bindings built out-of-tree, e.g. via OPAM, should append
a line to META.llvm like the following:

linkopts = "-cclib -L$libdir -cclib -Wl,-rpath,$libdir"

where $libdir is the lib/ directory where LLVM libraries are
installed.

llvm-svn: 221139