testsuite/ix86: prune MMX ABI warning

So far on 32-bit hosts this test failed (for both C and C++) because of
the ABI change warning occurring without (explictly) enabling MMX.

gcc/testsuite/

	* c-c++-common/torture/builtin-shufflevector-2.c: Prune ix86 MMX
	ABI warning.

gcc/testsuite/c-c++-common/torture/builtin-shufflevector-2.c

@@ -24,3 +24,6 @@ main (void)
       __builtin_abort ();
   return 0;
 }
+
+// On i?86-*-* an ABI warning would actually surface.
+// { dg-prune-output "MMX vector (argument|return) without MMX enabled changes the ABI" }