kern/sched_shims.c: back to ifunc
Reported by: kevans
Reviewed by: kevans, mhorne
Fixes: 0d3652f67d246348e2c017205c6782caf4484449
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differrential revision: https://reviews.freebsd.org/D55490
(cherry picked from commit ef1218add1d3d2baca9d3b8bcfb57e05ef6fc503)