apic: prevent divide by zero in CPU frequency init

apic: prevent divide by zero in CPU frequency init

If a CPU for some reason returns 0 as CPU frequency, we currently panic
on the resulting divide by zero when trying to initialize the CPU(s) via
APIC. When this happens, we'll fallback to measuring the frequency
instead.

PR: 269767
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/664
Approved by: re@ (cperciva)

(cherry picked from commit 499171a98c8813e4dc6e085461d5c47750efa555)
(cherry picked from commit a1c63b8e29e832d9ada7c252e7e80ce8b94aeb41)