Darwin, Arm64 : Account for stack addresses less aligned than DI.

darwinpcs, packs some stack items, which means that one cannot
guarantee that they are aligned to DI.  Check for these cases and
reject PRFM instructions then.

Note, that this generally results in use of an extra temporary reg.

clang uses 'PRFUM' instructions in those cases, so we have a missed
optimisation opportunity (low priority).

fixes issue #16.

gcc/config/aarch64/aarch64.c

@@ -9531,6 +9531,10 @@ aarch64_address_valid_for_prefetch_p (rtx x, bool strict_p)
   if (!res)
     return false;
 
+  /* Darwinpcs allows addresses on the stack that are not DImode aligned.  */
+  if (TARGET_MACHO && addr.offset && (INTVAL (addr.offset) & 0x07))
+    return false;
+
   /* ... except writeback forms.  */
   return addr.type != ADDRESS_REG_WB;
 }