RFC libstdc++-v3 -> libstdc++ rename

[Benjamin Kosnik]

> If nothing else, it would mean we could add an entry to the seemingly dead "news" at http://gcc.gnu.org/libstdc++/   ;-)

Ugh. That is such an embarrassment. Honestly, I think we should just
kill the news part of the libstdc++ homepage, or add in links to the
various release notes. Killing it seems easier.
 
> The docs would need updating in quite a few places but that's trivial.

Yep. Also, there would have to be some make/configure smoothing. But
that too would be largely mechanical.

-benjamin