[Patch] Fix libstdc++/11378 (take2) + xsputn optmizations

[Paolo Carlini]

Andreas Jaeger wrote:

>I would not use int64_t - let's use the libc types directly like
>fpos64_t and off64_t - and then using fsetpos64/fseeko64.
>
However, Andreas, please correct me as soon as possible if I'm wrong, 
mixing fseek and read and write (vs fread, fwrite) is not possible. We 
want non blocking read and write as our low level I/O primitives. Do you 
expect troubles with lseek64?

Paolo.