cksum: use more efficient slice by 8 algorithm

cksum: use more efficient slice by 8 algorithm

A 100MB file improves from 2.50s to 1.80s on a Sparc T5220
A 100MB file improves from 0.54s to 0.13s on an i3-2310M

* bootstrap.conf: Explicitly depend on byteswap,
since now used directly by coreutils.
* src/cksum.c (cksum): Process in multiples of 8 bytes.
(main): Adjust for generation of expanded crctab.
* src/cksum.h: Split now larger crctab to separate header.
* src/local.mk: Reference the new header.
* NEWS: Mention the improvement.