Fix one of the (larger) FIXMEs where we were misusing the Driver's idea

of the target triple to stand in for the "host" triple.

Thanks to a great conversation with Richard Smith, I'm now much more
confident in how this is proceeding. In all of the places where we
currently reason about the "host" architecture or triple, what we really
want to reason about in the detected GCC installation architecture or
triple, and the ways in which that differs from the target. When we find
a GCC installation with a different triple from our target *but capable
of targeting our target* through an option such as '-m64', we want to
detect *that* case and change the paths within the GCC installation (and
libstdc++ installation) to reflect this difference.

This patch makes one function do this correctly. Subsequent commits will
hoist the logic used here into the GCCInstallation utility, and then
reuse it through the rest of the toolchains to fix the remaining places
where this is currently happening.

llvm-svn: 148852