A follow-up to r151482 which added support for powerpc and powerpc64

Debian multiarch libraries, this should in theory add support for those
platform's header search rules. I don't have a system to check this
with, so review appreciated. I've added the corresponding tests
referring to the debian multiarch tree.

We are starting to have a relatively completely tested Linux platform
for header search and library search, with several interesting
peculiarities. We should point people at the debian_multiarch_tree when
suggesting new tests. Folks with Debian systems that can check this for
correctness, it would be much appreciated. The missing chunks I know of
are testing bi-arch peudo-cross-compiling toolchains betwen 32-bit and
64-bit variants of platforms, and the MIPS and ARM Debian toolchains.

llvm-svn: 151484