[CMake] Add $ORIGIN/../../../../lib to rpath if BUILD_SHARED_LIBS or LLVM_LINK_LLVM_DYLIB on *nix

Summary:
lib/python2.7/dist-packages/lldb/_lldb.so is a symlink to lib/liblldb.so,
which depends on lib/libLLVM*.so (-DBUILD_SHARED_LIBS=ON) or lib/libLLVM-10git.so
(-DLLVM_LINK_LLVM_DYLIB=ON). Add an additional rpath `$ORIGIN/../../../../lib` so
that _lldb.so can be loaded from Python.

This fixes an import error from lib/python2.7/dist-packages/lldb/__init__.py

  from . import _lldb
  ImportError: libLLVMAArch64CodeGen.so.10git: cannot open shared object file: No such file or directory

 The following configurations will work:

* -DBUILD_SHARED_LIBS=ON
* -DBUILD_SHARED_LIBS=OFF -DLLVM_LINK_LLVM_DYLIB=ON
* -DBUILD_SHARED_LIBS=OFF -DLLVM_LINK_LLVM_DYLIB=ON -DCLANG_LINK_CLANG_DYLIB=ON
  (-DCLANG_LINK_CLANG_DYLIB=ON depends on -DLLVM_LINK_LLVM_DYLIB=ON)

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D71800