libstdc++: Add { target c++98_only } to tests

These test behaviour only seen with -std=c++03 so the target selector
should match.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/bitset/107037.cc: Add c++98_only selector.
	* testsuite/26_numerics/complex/56111.cc: Likewise.

libstdc++-v3/testsuite/20_util/bitset/107037.cc

@@ -1,5 +1,5 @@
 // { dg-options "-std=c++03" }
-// { dg-do compile }
+// { dg-do compile { target c++98_only } }
 // PR libstdc++/107037 bitset::_M_do_reset fails for strict -std=c++03 mode
 #include <bitset>
 template class std::bitset<0>;

libstdc++-v3/testsuite/26_numerics/complex/56111.cc

@@ -1,5 +1,5 @@
-// { dg-do compile }
 // { dg-options "-std=c++98" }
+// { dg-do compile { target c++98_only } }
 
 // Copyright (C) 2013-2023 Free Software Foundation, Inc.
 //