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

[Paolo Carlini]

Nathan Myers wrote:

>I think it would be a grave error to introduce compile-time
>dependencies into the interface.  Glibc had no choice, as they
>were constrained by backward compatibility.  
>
>Instead, our streamoff and streampos types should always be 64-bit
>types on all platforms.  (That is, "int64_t", not "long long".) 
>Furthermore, we should use the 64-bit versions of system calls 
>automatically wherever they exist.  Where they don't, we should check 
>for range errors where we convert a 64-bit offset to a 32-bit offset.
>
>As a matter of staging, we might introduce the 64-bit types and the 
>range checks first, and then work on enabling use of the 64-bit system 
>calls after.  That would probably flush out helpers quicker.
>  
>
Thanks Nathan, for your feedback.

To be honest, when I first discussed a bit this issue with Andreas (a 64 
bit expert) in Ottawa he mentioned that the approach using compile-time 
dependencies was only one possibility, to be weighed against others. At 
the time I didn't fully understand what he meant, but now I do much better.

Paolo.