Find a better compromise with the default library search paths used by
Clang when linking and using a GCC installation from a GCC
cross-compiler.

This was desired already by two special case platforms (Android and
Mips), and turns out to be generally (if frustratingly) true. I've added
a substantial comment to the code clarifying the underlying assumptions
of doing actual cross compiles with Clang (or GCC for that matter!) and
help avoid further confusion here.

The end result is to realize that fully general form of PR12478 cannot
be resolved while we support existing cross-compiling GCC toolchains,
and linking with them (namely, linking against their libgcc and
libstdc++ installs). GCC installs these target libraries under
a target-specific prefix but one that may not be available within the
actual sysroot in use. When linking in this world, GCC works and Clang
should as well, but caveat emptor: DSOs from this tree must be
replicated and rpath-fixed to be found at runtime within the sysroot.

I've extended the cross compile test cases to cover these issues by
pointing them at a sysroot and actually checking the library search
paths.