Add 32-bit big-endian checksum function: cksum32

Implement the algorithm used by z/Architecture cksum instruction
(aka Internet 32-bit checksum).

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
s390x: Use VMCOREINFO from struct os_info if available

The os_info structure is more reliable, because the data is checksummed.
Note that LC_VMCORE_INFO is used as fallback.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Fix compilation on big-endian machines

I noticed that on the master branch there is a little typo:

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>