libstdc++: Add xfail to pretty printer tests that fail in C++20

For some reason the type printer for std::string doesn't work in C++20
mode, so std::basic_string<char, char_traits<char>, allocator<char> is
printed out in full rather than being shown as std::string. It's
probably related to the fact that the extern template declarations are
disabled for C++20, but I don't know why that affects GDB.

For now I'm just marking the relevant tests as XFAIL. That requires
adding support for target selectors to individual GDB directives such as
note-test and whatis-regexp-test.

libstdc++-v3/ChangeLog:

	* testsuite/lib/gdb-test.exp: Add target selector support to the
	dg-final directives.
	* testsuite/libstdc++-prettyprinters/80276.cc: Add xfail for
	C++20.
	* testsuite/libstdc++-prettyprinters/libfundts.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/prettyprinters.exp: Tweak
	comment.

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