Teach the logic for locating an installed GCC about the system root.

This requires fixing a latent bug -- if we used the default host triple
instead of an autodetected triple to locate GCC's installation, we
didn't go back and fix the GCC triple. Correct that with a pile of
hacks. This entire routine needs a major refactoring which I'm saving
for a subsequent commit. Essentially, the detection of the GCC triple
should be hoisted into the same routine as we locate the GCC
installation: the first is intrinsically tied to the latter. Then the
routine will just return the triple and base directory.

Also start to bring the rest of the library search path logic under
test, including locating crtbegin.o. Still need to test the multilib and
other behaviors, but there are also bugs in the way of that.

llvm-svn: 140995