Update a comment to clarify that searching the target triple bin

directory for programs used by the driver is actually the standard
behavior we want to be compatible with GCC cross compilers -- it isn't
specific to SUSE or any other distro.

Also start fleshing out testing of the different cross compilation
patterns, both with a new very bare-bones tree of cross compilers and by
extending the multilib trees. Currently, we don't correctly model doing
a cross compile using the non-triple target of a bi-arch GCC install,
but I'll add support for that (and tests) next.

llvm-svn: 184499