Move _Thread_queue_Queue::_Lock for RTEMS

Move _Thread_queue_Queue::_Lock for RTEMS

Move _Thread_queue_Queue::_Lock to begin of the structure.  On RTEMS,
the presence of a lock component in the thread queue structures actually
depends on the build-time RTEMS_SMP configuration option.  A move of
this part to the begin of the structure allows an implementation re-use
for the other parts.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>