sched_shim: restore kern.ccpu sysctl

It is apparently should be considered part of the ABI, and is used by
the base top(1). But do not declare the ccpu variable in headers, it is
needed only by 4bsd. So put the variable definition into sched_shim.c to
make the kernel buildable without SCHED_4BSD.

Reviewed by: olce
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D54831