Disable some libstdc++ tests for Windows text-mode I/O

[Paolo Carlini]

Joseph S. Myers wrote:
> This patch adds dg-require-binary-io to disable those tests if
> _GLIBCXX_HAVE_DOS_BASED_FILESYSTEM (the relevant condition used in
> showmanyc).  Tested with cross to i686-mingw32.  OK to commit?
>   
Yes, it's ok for now thanks (probably at some point we want to split out
from those tests the bits sensitive to the implementation of showmanyc)

Paolo.