This effective target means we can skip/xfail tests that depend on the
new std::string, std::list etc.

I've added the check_effective_target_cxx11 procedure in libstdc++.exp
not in gcc/testsuite/lib/target-supports.exp because I don't think it
is useful outside the libstdc++ testsuite.

There are still lots of tests that fail when run with
-D_GLIBCXX_USE_CXX11_ABI=0 but I plan to fix them by adding the
missing features, not skipping the tests.

	PR libstdc++/79075
	* testsuite/lib/libstdc++.exp (check_v3_target_filesystem_ts): Remove
	redundant option from cxxflags.
	(check_effective_target_cxx11-abi): Define.
	* testsuite/21_strings/basic_string/allocator/71964.cc: Use cxx11-abi
	effective target.
	* testsuite/21_strings/basic_string/allocator/char/copy.cc: Likewise.
	* testsuite/21_strings/basic_string/allocator/char/copy_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/minimal.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/move.cc: Likewise.
	* testsuite/21_strings/basic_string/allocator/char/move_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/noexcept.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/swap.cc: Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/minimal.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/move.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/move_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/noexcept.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/swap.cc:
	Likewise.
	* testsuite/23_containers/list/61347.cc: Likewise.
	* testsuite/27_io/basic_fstream/cons/base.cc: Likewise.
	* testsuite/27_io/ios_base/failure/cxx11.cc: Likewise.

Tested powerpc64le-linux, committed to trunk.