merge from stable, revs:

  1.74.4.1  +12 -11    fptools/ghc/compiler/nativeGen/MachCode.lhs
  1.30.4.1  +23 -0     fptools/ghc/compiler/nativeGen/Stix.lhs
  1.70.4.1  +2 -5      fptools/ghc/compiler/nativeGen/StixPrim.lhs

  Route all NCG panics to do with missing primop implementations and any
  other panic which could be caused by compiling legitimate sources
  through the function Stix.ncgPrimopMoan.  This emits a helpful message
  explaining what has happened, advises the use of -fvia-C as a
  workaround, and says please mail us.