add MO_WriteBarrier to CallishMachOps

This will let us express write barriers in C--

compiler/cmm/MachOp.hs

@@ -385,6 +385,7 @@ data CallishMachOp
   | MO_F32_Log
   | MO_F32_Exp
   | MO_F32_Sqrt
+  | MO_WriteBarrier
   deriving (Eq, Show)
 
 pprCallishMachOp :: CallishMachOp -> SDoc

compiler/cmm/PprC.hs

@@ -547,6 +547,7 @@ pprCallishMachOp_for_C mop
         MO_F32_Log  -> ptext SLIT("logf")
         MO_F32_Exp  -> ptext SLIT("expf")
         MO_F32_Sqrt -> ptext SLIT("sqrtf")
+	MO_WriteBarrier -> ptext SLIT("write_barrier")
 
 -- ---------------------------------------------------------------------
 -- Useful #defines

compiler/nativeGen/MachCodeGen.hs

@@ -2909,6 +2909,10 @@ genCCall fn cconv result_regs args
 
 #if i386_TARGET_ARCH
 
+genCCall (CmmPrim MO_WriteBarrier) _ _ _ = return nilOL
+	-- write barrier compiles to no code on x86/x86-64; 
+	-- we keep it this long in order to prevent earlier optimisations.
+
 -- we only cope with a single result for foreign calls
 genCCall (CmmPrim op) [(r,_)] args vols = do
   case op of
@@ -3119,6 +3123,10 @@ outOfLineFloatOp mop res args vols
 
 #if x86_64_TARGET_ARCH
 
+genCCall (CmmPrim MO_WriteBarrier) _ _ _ = return nilOL
+	-- write barrier compiles to no code on x86/x86-64; 
+	-- we keep it this long in order to prevent earlier optimisations.
+
 genCCall (CmmPrim op) [(r,_)] args vols = 
   outOfLineFloatOp op r args vols