[libcxx]handling missing linux implementations

It would be good to also keep Windows in mind. I think a lot of people
would like to see libcxx on Windows!

If there are Linux-Windows compatible CRT/glibc implementations, I
think it would be profitable for both platforms (and perhaps even for
Mac) to have a common implementation. It seems most locale stuff is
compatible enough (ignoring function and macro naming) to write common
code.

Ruben