[committed] Adjust ARC tests after c23 changes

This test passes different kinds of objects to the underlying function.  So
just pass in -std=gnu17.

gcc/testsuite
	* gcc.target/arc/add_n-combine.c: Pass in -std=gnu17.

gcc/testsuite/gcc.target/arc/add_n-combine.c

@@ -1,5 +1,5 @@
 /* { dg-do compile }  */
-/* { dg-options "-O2" }  */
+/* { dg-options "-O2 -std=gnu17" }  */
 
 struct b1 {
       char c;