Kthreads can share the group data so that to reduce overheads. This implements shared kthread group via: - use `tcb_s` instead of `task_tcb_s` for kthreads - use `g_kthread_group` when creating kthreads - use stackargs to start tasks and kthreads see pull/12320 for test logs. Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>