Reword and reformat some of the "Multiarch" code in the toolchain setup.

There are fundamentally two different things that were getting conflated
here.

1) A bi-arch GCC toolchain install. This is not a full blown cross
   compiler, but it supports targetting both 32-bit and 64-bit variants
   of the same architecture using multilib OS installs and runtimes.

2) A "multiarch" Debian OS/runtime layout that lays out the libraries,
   headers, etc as-if there were going to be a full blown cross compiler
   even when in reality it is just a bi-arch GCC targeting two variants.
   Also, these tend to use oddly "canonicalized" triples without the
   vendor in them unlike the typical cross compiler runtime library
   search that vanilla GCC cross compilers perform.

Now, when we mean the bi-arch nature of GCC accomplished with just
a suffix or tweak to the GCC paths, we say 'Biarch' or something
related. When we mean the Debian layout of includes and libraries, we
say multiarch or reference the multiarch triple.

In the process of reading and often renaming stuff in all these places,
also reformat with clang-format. No functionality change should be going
on here, this is just tidying up.

llvm-svn: 184632