[Bug libstdc++/53683] cout doesn't support std::u16string

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53683

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We also have <ext/codecvt_specializations.h> but it needs a lot of work to
re-use.