[patch libstdc++]: Add some missing errno-constants for mingw-targets

[Paolo Carlini]

Hi,
> Hello,
>
> this patch adds some errno values supported by newer runtime-version.
> To keep backward compatibility
> this patch checks for their definition before trying to use them.
Any particular reason for not using [GLIBCXX_CHECK_SYSTEM_ERROR] for 
mingw too (by extending the list of checked syserr appropriately)?

Paolo.