This fixes building against cygwin which does not define off64_t. off_t
under modern versions of OS X(or macOS) defaults to 64-bit. Furthermore,
_FILE_OFFSET_BITS is defined to 64-bit which eliminates the need for
using off64_t directly. Also, musl just like OS X defines off_t as 64-bit
in all situations. Also removed some code related to this.
Signed-off by: Rosen Penev <rosenp@gmail.com>
