hwpmc: formatting of CPU and class lists

The end result is much more legible in both cases.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39906

(cherry picked from commit 7253dc57a0b400ff745a43017928304ff8dcf33f)