make-bootstrap-tools.darwin: upgrade to llvm 3.8

This will break part of the bootstrap tools tests because the new tools
need some changes in the stdenv, but if I change them all at once, the
stdenv breaks with the old bootstrap tools. So I'm doing this first, then
will make changes to the stdenv once this bundle is built and I can use
it.

I also added some functionality to let me test one set of bootstrap tools
on another nixpkgs tree, which makes testing a lot more pleasant.