KDB: restart only CPUs stopped by KDB

There is a case when not all CPUs went online. In that situation,
restart only APs which were operational before entering KDB.

Created by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Reviewed by: nwhitehorn
Differential revision: https://reviews.freebsd.org/D13949
Sponsored by: QCM Technologies