Fix up the sparc native code generator.  Mostly dull stuff.  Notable
changes:

* Cleaned up ccall mechanism for sparc somewhat.

* Rearranged assignment of sparc floating point registers (includes/MachRegs.h)
  so the NCG's register allocator can handle the double-single pairing
  issue without modification.  Split VirtualRegF into VirtualRegF and
  VirtualRegD, and split RcFloating into RcFloat and RcDouble.  Net effect
  is that there are now three register classes -- int, float and double,
  and we pretend that sparc has some float and some double real regs.

* (A fix for all platforms): propagate MachFloats through as StFloats,
  not StDoubles.  Amazingly, until now literal floats had been converted
  to and treated as doubles, including in ccalls.