Add support for PPC and PPC64 multiarch toolchains on Debain.

Patch from Michel Dänzer, sent our way via Jeremy Huddleston who added
64-bit support. I just added one other place where powerpc64-linux-gnu
was missing (we only had powerpc64-unknown-linux-gnu).

I've also added a tree to test out the debian multiarch stuff. I don't
use debian regularly, so I'm not certain this is entirely accurate. If
anyone wants to check it against a debian system and fix any
inaccuracies, fire away. This way at least folks can see how this is
*supposed* to be tested.

It'd be particularly good to get the Debian MIPS toolchains tested in
this way.

llvm-svn: 151482