tools/sched_ext: scx_qmap: Add cmask-based idle tracking and cid-based idle pick

Switch qmap's idle-cpu picker from scx_bpf_pick_idle_cpu() to a
BPF-side bitmap scan, still under cpu-form struct_ops. qa_idle_cids
tracks idle cids (updated in update_idle / cpu_offline) and each
task's taskc->cpus_allowed tracks its allowed cids (built in
set_cpumask / init_task); select_cpu / enqueue scan the intersection
for an idle cid. Callbacks translate cpu <-> cid on entry;
cid-qmap-port drops those translations.

The scan is barebone - no core preference or other topology-aware
picks like the in-kernel picker - but qmap is a demo and this is
enough to exercise the plumbing.

v3: qmap_init() refuses to load when nr_cids exceeds SCX_QMAP_MAX_CPUS;
    task_ctx's flex array would otherwise overflow into the next slab
    entry. (Sashiko)

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Reviewed-by: Changwoo Min <changwoo@igalia.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

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