sched_ext: Introduce LLC awareness to the default idle selection policy

Rely on the scheduler topology information to implement basic LLC
awareness in the sched_ext build-in idle selection policy.

This allows schedulers using the built-in policy to make more informed
decisions when selecting an idle CPU in systems with multiple LLCs, such
as NUMA systems or chiplet-based architectures, and it helps keep tasks
within the same LLC domain, thereby improving cache locality.

For efficiency, LLC awareness is applied only to tasks that can run on
all the CPUs in the system for now. If a task's affinity is modified
from user space, it's the responsibility of user space to choose the
appropriate optimized scheduling domain.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>