[OCaml] Add an ocamlfind package llvm.all_backends.

This package is useful for architecture-independent tools like llc.

Patch by Jacques-Pascal Deplaix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207793 91177308-0d34-0410-b5e6-96231b3b80d8