Teach Clang to look in its installation libdir for libraries (such as
libc++) when the installation is within the system root.

This doesn't really help cross compiles much, but we don't (currently)
have a great story around libc++, cross compiles, and who is responsible
for building and/or installing the libraries. However, it handles the
very common case of non-cross builds in a way entirely consistent with
GCC, so I'm hopeful this won't really hose anyone.

This is the second patch that I think should be backported to 3.4 to
give folks an easy to checkout and install working Clang+libc++
toolchain.