[libcxx] Make sure use_system_cxx_lib does not override cxx_runtime_root for DYLD_LIBRARY_PATH

Otherwise, even specifying a runtime root different from the library
we're linking against won't work -- the library we're linking against
is always used. This is undesirable if we try testing something like
linking against a recent libc++.dylib but running the tests against an
older version (the back-deployment use case).

llvm-svn: 349171