[OCaml] Clear cross-target test deps when building out-of-tree

Clear cross-target test dependencies when using LLVM_OCAML_OUT_OF_TREE,
in order to make it possible to run check-llvm-bindings-ocaml without
rebuilding the whole LLVM.

Differential Revision: https://reviews.llvm.org/D26580

llvm-svn: 286720