Re-introduce kern.sched.topology_spec

Move it back from kern.sched.ule.topology_spec.
Make it scheduler-agnostic.
Provide trivial report for UP kernels.

Apparently the MIB is used by some third-party software. Obviously it
did not worked on UP or 4BSD configs.

PR: 292574
Reviewed by: olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D55062