[CMake] Fix rpath construction for out-of-tree builds

This patch was produced in conjunction with Michał Górny. It should resolve the issues that were trying to be solved by D25304.

This moves rpath handling into `llvm_add_library` and `add_llvm_executable` so that it is available to all projects using AddLLVM whether built in-tree or out-of-tree.

llvm-svn: 285714