Retire notion of target.

[jaykrell]

I think with the C++ backend and maybe LLVM, we can remove the notion of target.
I think users should create an output directory and we should run autoconf or cmake or config.guess,
in order to pass on the LLVM triple and/or word space and endian.

Even with the gcc backend I don't think target is motivated.

Target is really just an output directory name and a normalized config.guess output.