identcpu: Let the hw.model be readable in capability mode

On some platforms this static information can be derived directly from
the CPU, and there doesn't seem much use in restricting it.

Reviewed by: emaste, imp, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43281

(cherry picked from commit 0aba5dd2af57ea3ae90eb49d7745c4c5b6f0ab6b)