Add an unreachable() in the BUG() implementations, to get rid of
several warnings similar to the one below:

 kernel/sched/core.c: In function 'pick_next_task':
 kernel/sched/core.c:2690:1: warning: control reaches end of non-void function [-Wreturn-type]
Signed-off-by: NRabin Vincent <rabin@rab.in>
Signed-off-by: NJesper Nilsson <jespern@axis.com>