fix(cksum): add debug option and resolve GNU cksum.sh --debug option test

[karanabe]

Implement cksum --debug option by mirroring GNU’s hardware reporting (including GLIBC_TUNABLES handling), documenting the helper, adding localized help text, and extending the cksum tests to cover the new flag and hwcap parser.

#9127

[sylvestre]

dupe of #9088 ?

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cksum/cksum is no longer failing!

[karanabe]

dupe of #9088 ?

Sorry, I missed that earlier.
I’ll keep this open for a while in case it’s useful or can be referenced.
I plan to close this PR a bit later.