Unify and cleanup rpath handling in tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223594 91177308-0d34-0410-b5e6-96231b3b80d8
Fix apple clang detection in lit.cfg

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223597 91177308-0d34-0410-b5e6-96231b3b80d8