Make the sparc native code generator work again after recent
primop hackery.

* Track the change from PrimOp to MachOp at the Stix level.

* Teach the sparc insn selector how to generate 64-bit code.

* Fix various bogons in sparc {Int,Double,Float} <-> {Int,Double,Float}
  conversions which only happened to generate correct code by
  accident, so far.

* Synthesise BaseReg from &MainCapability.r on archs which do not
  have BaseReg in a regiser (eg sparc :)

At the moment {add,sub,mul}Int# are not implemented.  To be fixed.