Test a multilib setup on a 64-bit host. This is the far more common

configuration, although the test still stubs out more directories than
are necessary or common in order to exercise all of the lookup paths
observed with upstream GCC.

This finishes testing the distribution-independent and
GCC-installation-independent parts of the library path search logic.
More testing is still needed for the triple detection, GCC-installation
detection, and handling distributions with unusual configurations.

llvm-svn: 141000