Update a comment to clarify that searching the target triple bin

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184499 91177308-0d34-0410-b5e6-96231b3b80d8