Broken links to libstdc++ docs

[Jonathan Wakely]

Once again the links from http://gcc.gnu.org/onlinedocs/ to the
libstdc++ docs don't work:

http://gcc.gnu.org/onlinedocs/gcc-4.6.4/libstdc++/manual/
http://gcc.gnu.org/onlinedocs/gcc-4.7.3/libstdc++/manual/