Fix ass to match the EM spec.

The spec says, "ASS w: Adjust the stack pointer by w-byte integer".
The w argument "can either be given as argument or on top of the
stack."  Therefore, 'ass 4' would pop the 4-byte integer from the
stack, but 'ass' would pop the size w from the stack, then pop the
w-byte integer.

PowerPC ncg wrongly implemented 'ass' as if it was 'ass 4'.  Fix it to
accept only 'ass 4'.

mach/powerpc/ncg/table

@@ -2003,14 +2003,14 @@ PATTERNS
 			gen
 				move %1, SP
 
-	pat loc ass $1==4                  /* Drop 4 bytes from stack */
+	pat loc ass $1==4 && $2==4         /* Drop 4 bytes from stack */
 		with exact GPR
 			/* nop */
 		with STACK
 			gen
 				addi SP, SP, {CONST, 4}
 
-	pat ass                            /* Adjust stack by variable amount */
+	pat ass $1==4                      /* Adjust stack by variable amount */
 		with CONST2 STACK
 			gen
 				move {SUM_RC, SP, %1.val}, SP
@@ -2028,7 +2028,7 @@ PATTERNS
 	pat asp                            /* Adjust stack by constant amount */
 		leaving
 			loc $1
-			ass
+			ass 4