In preparation for switching linuxulator to the use the native 1:1
threads refactor kern_sched_rr_get_interval() and sys_sched_rr_get_interval().
Add a kern_sched_rr_get_interval() counterpart which takes a targettd
parameter to allow specify target thread directly by callee (new Linuxulator).

Linuxulator temporarily uses first thread in proc.

Move linux_sched_rr_get_interval() to the MI part.

Differential Revision: https://reviews.freebsd.org/D1032
Reviewed by: trasz