sched/fair: Relax task_hot() for misfit tasks

Misfit tasks can and will be preempted by the stopper to migrate them over
to a higher-capacity CPU. However, when not-currently-running misfit tasks
are scanned by the load balancer (i.e. detach_tasks()), the task_hot()
ratelimiting logic may prevent us from enqueuing said task onto a
higher-capacity CPU.

Align detach_tasks() with the active-balance logic and let it pick a
cache-hot misfit task when the destination CPU can provide a capacity
uplift.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>