[dev.power64] cmd/cc: round argsize to multiples of wordsize on power64

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/121640043

src/cmd/cc/pgen.c

@@ -89,7 +89,7 @@ argsize(void)
 		}
 //print("	%d %T\n", s, t);
 	}
-	if(thechar == '6')
+	if(thechar == '6' || thechar == '9')
 		s = (s+7) & ~7;
 	else
 		s = (s+3) & ~3;