libstdc++: Fix gdb pretty printers when dealing with std::string

Since revision 33b43b0d std::string and other
similar typedef are ambiguous from a gdb point of view because it matches both
std::basic_string<char> and std::__cxx11::basic_string<char> symbols. For those
typedef add a workaround to accept the substitution as long as the same regardless
of __cxx11 namespace.

Also avoid to register printers for types in std::__cxx11::__8:: namespace, there is
no such symbols.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (Printer.add_version): Do not add version
	namespace for __cxx11 symbols.
	(add_one_template_type_printer): Likewise.
	(add_one_type_printer): Likewise.
	(FilteringTypePrinter._recognizer.recognize): Add a workaround for std::string & al
	ambiguous typedef matching both std:: and std::__cxx11:: symbols.
	* testsuite/libstdc++-prettyprinters/cxx17.cc: Remove obsolete
	\#define _GLIBCXX_USE_CXX11_ABI 0.
	* testsuite/libstdc++-prettyprinters/simple.cc: Likewise. Adapt test to accept
	std::__cxx11::list.
	* testsuite/libstdc++-prettyprinters/simple11.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/whatis.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/80276.cc: Likewise and remove xfail for c++20
	and debug mode.
	* testsuite/libstdc++-prettyprinters/libfundts.cc: Likewise.