[SystemZ] Rename and formatting fixes

In hindsight, using "RISBG" for something that can be any type of
R.SBG instruction was a bit confusing, so this renames it to RxSBG.
That might not be the best choice either, since there is an instruction
called RXSBG, but hopefully the lower-case letter stands out enough.

While there I fixed a couple of GNUisms that had crept in --
sorry about that!