Add stubs for the new aio_*() system calls.

This change adds stubs for the asynchronous I/O system calls including:
aio_cancel(2), aio_error(2), aio_return(2), aio_read(2), aio_write(2),
aio_syspend(2), aio_waitcomplete(2), aio_fsync(2), and aio_mlock(2).