Add kern/sched_shim.c

This is the infrastructure to allow scheduler implementation to be
selected on boot, supported by ifuncs.

The DEFINE_SHIM() macros and their usage provided by jrtc27@.

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