[OCaml] Install dynamic libraries in 'stubdirs' directory

Install the OCaml dynamic libraries in the 'stubdirs' directory rather
than the llvm subdirectory in order to fix running executables created
by ocamlc. Otherwise, the executables fail to run being unable to locate
the libraries (unless the LLVM directory is explicitly added to
LD_LIBRARY_PATH).

The staging directories are not altered since they work for our
development setup anyway, and installing into two directories would
unnecessarily make the code more complex.

Differential Revision: https://reviews.llvm.org/D35995

llvm-svn: 309481