x86/mp: don't create empty cpu groups

When some APICs are disabled by tunables, some cpu groups could end up
empty. An empty cpu group causes the system to panic because not all
functions handle them correctly. Additionally, it's wasted time to
handle and inspect empty cpu groups. Therefore, just don't create them.

Reviewed by: kib, avg, cem
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24927