libstdc++: XFAIL tests that depends on RTTI

libstdc++: XFAIL tests that depends on RTTI

The std::emit_on_flush manipulator depends on dynamic_cast, so fails
without RTTI.

The std::async code can't catch a forced_unwind exception when RTTI is
disabled, so it can't rethrow it either, and the test aborts.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_ostream/emit/1.cc: Expect test to fail
	if -fno-rtti is used.
	* testsuite/30_threads/async/forced_unwind.cc: Expect test
	to abort if -fno-rtti is used.