sched_ext: Add scx_cpu0 example scheduler

sched_ext: Add scx_cpu0 example scheduler

Add scx_cpu0, a simple scheduler that queues all tasks to a single DSQ and
only dispatches them from CPU0 in FIFO order. This is useful for testing bypass
behavior when many tasks are concentrated on a single CPU. If the load balancer
doesn't work, bypass mode can trigger task hangs or RCU stalls as the queue is
long and there's only one CPU working on it.

v2: Check whether task is on CPU0 at enqueue using scx_bpf_task_cpu() instead
    of nr_cpus_allowed (Andrea Righi).

Cc: Dan Schatzberg <schatzberg.dan@gmail.com>
Cc: Emil Tsalapatis <etsal@meta.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>