Add a new Hurd OS for Triple, and recognize it from "gnu0" prefixes
(coming from the typical triple, e.g. "i686-unknown-gnuX.Y").
Recognizing the unversioned OS name "gnu" would create too much
confusion with the "gnu" environment, so that is skipped for now.
Also, tweak the normalization test in TripleTest: avoid checking
permutations of the Hurd OS, as it could be mishandled as environment.