rustc::back: fail if a valid triple can't be found

This is a fairly heavy handed solution to the underlying problem.

As with a lot of my PR's, I'm hoping for a sanity check before I invest a bunch of time; I believe that `create_target_machine` should return `Result<TargetMachineRef, Err>`, but pushing that change through all the calling frames looks a bit painful (there were ~18 frames on this fault), and it's not an error that can be dealt with gracefully, modulo printing a nicer message.

Thoughts?