Drop SPARC NCG and associated paraphernalia
As noted in #16882 (closed), GHC's SPARC support is quite bitrotted and we currently have no ability to test it. This branch removes it and cleans up some of the vestiges which SPARC had kept alive (namely, RealReg
's RegPair
constructor).
Closes #16882 (closed). Closes #17602 (closed).