Move kernel stack allocation from exec_module() (binary format execution) to
nxtask_init() (task initialization) with proper type checking. Add conditional
check to allocate kernel stack only for non-kernel tasks (ttype != TCB_FLAG_TTYPE_KERNEL).
Enables kernel stack allocation for both exec'd tasks and regular task_create()
calls in protected builds, allowing userspace to create tasks with proper kernel
stack isolation.
Signed-off-by: hujun5 <hujun5@xiaomi.com>