libstdc++: Disable std::forward_list tests for C++98 mode

These tests fail with -std=gnu++98/-D_GLIBCXX_DEBUG in the runtest
flags. They should require the c++11 effective target.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/forward_list/debug/iterator1_neg.cc:
	Skip as UNSUPPORTED for C++98 mode.
	* testsuite/23_containers/forward_list/debug/iterator3_neg.cc:
	Likewise.

libstdc++-v3/testsuite/23_containers/forward_list/debug/iterator1_neg.cc

@@ -15,7 +15,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 //
-// { dg-do run { xfail *-*-* } }
+// { dg-do run { target c++11 xfail *-*-* } }
 // { dg-require-debug-mode "" }
 
 #include <forward_list>

libstdc++-v3/testsuite/23_containers/forward_list/debug/iterator3_neg.cc

@@ -15,7 +15,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 //
-// { dg-do run { xfail *-*-* } }
+// { dg-do run { target c++11 xfail *-*-* } }
 // { dg-require-debug-mode "" }
 
 #include <forward_list>