sched: Force the address order of each sched class descriptor

In order to make a micro optimization in pick_next_task(), the order of the
sched class descriptor address must be in the same order as their priority
to each other. That is:

 &idle_sched_class < &fair_sched_class < &rt_sched_class <
 &dl_sched_class < &stop_sched_class

In order to guarantee this order of the sched class descriptors, add each
one into their own data section and force the order in the linker script.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/157675913272.349305.8936736338884044103.stgit@localhost.localdomain

kernel/sched/idle.c

@@ -453,7 +453,8 @@ static void update_curr_idle(struct rq *rq)
 /*
  * Simple, special scheduling class for the per-CPU idle tasks:
  */
-const struct sched_class idle_sched_class = {
+const struct sched_class idle_sched_class
+	__attribute__((section("__idle_sched_class"))) = {
 	/* .next is NULL */
 	/* no enqueue/yield_task for idle tasks */