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

[Pétur Runólfsson]

Andreas Jaeger wrote:
> You can use lseek64 and off64_t, there should be no problem (if lseek
> is part of the C library).

Is off64_t available on all platforms supported by gcc? Otherwise it's
probably better to use int64_t everywhere (also on platforms that
don't have large file support now).

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.

Petur