FYI: update libstdc++ pretty-printers

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tromey@redhat.com> writes:

Tom> Doug Evans recently added some features to gdb to make it possible for
Tom> the user to individually enable or disable pretty-printers.  In order
Tom> for this to work, the printers must conform to a certain extended
Tom> protocol.

Tom> I happened to notice that I forgot to actually check the 'enabled'
Tom> flag -- so disabling did not actually work with the previous patch.

Tom> Here's an updated patch.

Now that GCC is in Stage 1, I am checking this in.

Tom