sched_ext: improve WAKE_SYNC behavior for default idle CPU selection

In the sched_ext built-in idle CPU selection logic, when handling a
WF_SYNC wakeup, we always attempt to migrate the task to the waker's
CPU, as the waker is expected to yield the CPU after waking the task.

However, it may be preferable to keep the task on its previous CPU if
the waker's CPU is cache-affine.

The same approach is also used by the fair class and in other scx
schedulers, like scx_rusty and scx_bpfland.

Therefore, apply the same logic to the built-in idle CPU selection
policy as well.

Signed-off-by: Andrea Righi <andrea.righi@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>