Darwin, testsuite: -multiply_defined is obsolete

gcc/testsuite/ChangeLog:

	* gcc.dg/ssp-2.c: Ignore warning that
	-multiply_defined is obsolete

gcc/testsuite/gcc.dg/ssp-2.c

 /* { dg-do run { target native } } */
 /* { dg-options "-fstack-protector" } */
 /* { dg-options "-fstack-protector -Wl,-multiply_defined,suppress" { target *-*-darwin* } } */
+/* { dg-prune-output "-multiply_defined is obsolete" } */
 /* { dg-require-effective-target fstack_protector } */
 
 #include <stdlib.h>