sys/syz-sysgen: check that const files are up-to-date

[dvyukov]

8aa6ae9 did not update const files, and as the result our downstream builds failed later.

We don't re-generate consts on CI b/c it requires a linux checkout, but at least syz-sysgen could check that .const files contain all required consts (and don't contain removed ones). It should catch most errors.

[tarasmadan]

What else is doable having the full linux checkout? I think we can do the checkout. At least for the merge-queue actions.

[dvyukov]

What else is doable having the full linux checkout? I think we can do the checkout. At least for the merge-queue actions.

#604 and #4022