[SystemZ/ZOS] Add binary format goff and operating system zos to the triple

Adds the binary format goff and the operating system zos to the triple
class. goff is selected as default binary format if zos is choosen as
operating system. No further functionality is added.

Reviewers: efriedma, tahonermann, hubert.reinterpertcast, MaskRay

Reviewed By: efriedma, tahonermann, hubert.reinterpertcast

Differential Revision: https://reviews.llvm.org/D82081