libstdc++: Use gdb.ValuePrinter base class

GDB 14 will add a new ValuePrinter tag class that will be used to
signal that pretty-printers will agree to the "extension protocol" --
essentially that they will follow some simple namespace rules, so that
GDB can add new methods over time.

A couple new methods have already been added to GDB, to support DAP.
While I haven't implemented these for any libstdc++ printers yet, this
patch makes the basic conversion: printers derive from
gdb.ValuePrinter if it is available, and all "non-standard" (that is,
not specified by GDB) members of the various value-printing classes
are renamed to have a leading underscore.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py: Use gdb.ValuePrinter
	everywhere.  Rename members to start with "_".