alarm: prepare for syscall interruption implementation

Move alarm cancellation logic to a separate function
cancel_alarm_ll(). This fucntion will be called on
thread destruction to avoid post-mortem alarms.

Also move alarm scheduling logic to a separate function
set_alarm_ll() to make code symmetric.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>