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

[Paolo Carlini]

Andreas Jaeger wrote:

>>The issue here is ABI stability; if a platform doesn't have large file
>>support now, but gets it in the future it should be possible to enable
>>large file support in basic_filebuf without breaking the ABI.
>>    
>>
>My point here is type safety, we should use the proper types and not
>invent some int64_t that works today but breaks in the future
>(remember when everybody used long instead of off_t - or even int?
>That makes it quite difficult to convert programs to off64_t).
>
First blush, I'm definitely on Andreas side, even if I see that Pétur 
has a point...

We should not stabilize a "dirty" (dunno if this is really good english, 
sorry), non type safe ABI for the sake of non-existant systems!

Paolo.