digest: add support for sm3

digest: add support for sm3

Add message digest sm3, which uses the OSCCA SM3 secure
hash (OSCCA GM/T 0004-2012 SM3) generic hash transformation.

* bootstrap.conf: Add the sm3 module.
* doc/coreutils.texi: Mention the cksum -a option.
* src/digest.c: Provide support for --algorithm='sm3'.
* tests/misc/sm3sum.pl: Add a new test (from Tianjia Zhang)
* tests/local.mk: Reference the new test.
* NEWS: Mention the new feature.

Tested-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>