[PATCH] libstdc++: use copy_file_range, improve sendfile in filesystem::copy_file

[Jannik Glückert]

The current copy_file implementation is suboptimal. It only uses
sendfile for files smaller than 2GB, falling back to a userspace copy,
and does not support copy_file_range at all.
copy_file_range is particularly of increasing importance with the
adoption of reflinks in filesystems.

I am pretty sure I got some of the formatting wrong, feel free to tear apart.
I don't know if sendfile has identical semantics on linux as it does
on solaris, if someone could test with a big file that'd be great.
Otherwise, this should not regress. The implementation will fall back
to sendfile / userspace copy if copy_file_range is not available for
the target paths.

The copy implementations for sendfile and copy_file_range were put
into separate functions and the callee code simplified to the point
where you can basically just copy-paste it to add a new
implementation, should new interesting syscalls pop up.

Best
Jannik
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