suppress gcc 8 aliasing warnings

function                                             old     new   delta
sigprocmask_SIG_SETMASK                                -      16     +16
wait_for_child_or_signal                             221     213      -8
dowait                                               432     424      -8

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -4217,7 +4217,7 @@ wait_block_or_sig(int *status)
 		/* Children exist, but none are ready. Sleep until interesting signal */
 #if 1
 		sigfillset(&mask);
-		sigprocmask(SIG_SETMASK, &mask, &mask);
+		sigprocmask_SIG_SETMASK(&mask); /* mask is updated */
 		while (!got_sigchld && !pending_sig)
 			sigsuspend(&mask);
 		sigprocmask(SIG_SETMASK, &mask, NULL);