tools/sched_ext: Add helper to check task migration state

commit 5f52bbf2 upstream.

Introduce a new helper for BPF schedulers to determine whether a task
can migrate or not (supporting both SMP and UP systems).

Fixes: e9fe1827 ("sched_ext: selftests/dsp_local_on: Fix sporadic failures")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>