kernel: make order_key field in thread conditional

The 'order_key' field in the thread structure '_thread_base' is only
required when CONFIG_SCHED_SCALABLE and/or CONFIG_WAITQ_SCALABLE are
enabled (neither of which is a default setting). Making the existence
of this field conditional slightly reduces the size of the k_thread
structure when neither of those Kconfig options are selected.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>