[wwwdocs] Update changes.html with libstdc++ changes

[Jonathan Wakely]

This adds recent libstdc++ updates to gcc-5/changes.html

I'm also noting one old change in the GCC 4.5 page, and
removing/changing some links to the C++0x status table. The list of
features supported on trunk is fairly irrelevant to someone looking at
the 4.4 release notes, so I've linked to the docs for the relevant
release, or just removed the link for 4.4 and 4.5 because we don't
have libstdc++ docs online for those releases.

Committed to CVS.
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