libstdc++/10167: ieee_1003.1-[3.2/3.3 regression] 2001 locale specialisations on a RedHat8.0/glibc-2.3.2-4 system

[Benjamin Kosnik]

>    Benjamin, at this point in the release cycle of 3.3 we
>    had better only fully qualifying (error as 
>    codecvt_base::error, etc.), instead of fully backporting
>    Pétur's rewrote, I think. Do you agree?
>    Thanks, Paolo.

Yes, just make sure it passes make check-abi.

thanks!
benjamin