SCHEDULER_STOPPED(): Move it (back) to 'systm.h'

It's not an assertion, so doesn't logically belong to 'kassert.h'.
Moreover, a subsequent commit will make it rely on a variable whose
declaration also belongs to 'systm.h'.

Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43571