kernel/sched: Refactor/simplify run queue accessors

Similar to the previous patch, the various _priq_run_*() functions are
always passed a first argument that is the singleton system run queue
(this is because the same backend functions are used by wait queues).

Refactor into a simpler API that places the access to the run queue in
just a single spot.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>