fix sendfile syscall to use 64-bit off_t

fix sendfile syscall to use 64-bit off_t

somehow the remapping of this syscall to the 64-bit version was
overlooked. the issue was found, and patch provided, by Stefan
Kristiansson. presumably the reason this bug was not caught earlier is
that the syscall takes a pointer to off_t rather than a value, so on
little-endian systems, everything appears to work as long as the
offset value fits in the low 31 bits. on big-endian systems, though,
sendfile was presumably completely non-functional.

corresponds to commit 55f45bc7222ec50b72aa8411c61e30184d0ade23 in
master branch.