Reallocate syscall numbers for aio_writev and aio_readv

The originally chosen numbers interfere with downstream projects'
syscalls. Move them to the end of the syscall table instead.

Reported by: jrtc27
Reviewed by: brooks
MFC-With: 022ca2fc7fe08d51f33a1d23a9be49e6d132914e
Differential Revision: 022ca2fc7fe08d51f33a1d23a9be49e6d132914e