[arch][arm-m] refactor systick code into separate module

Add generic support for running systick in monotonic mode and providing
current_time() routines.

Platforms may choose not to use this, thus it is moved into an optional
module.

arch/arm/arm-m/arch.c

@@ -82,8 +82,8 @@ void arch_early_init(void)
 	NVIC_SetPriority(SVCall_IRQn, arm_cm_lowest_priority());
 	NVIC_SetPriority(PendSV_IRQn, arm_cm_lowest_priority());
 
-	/* initialize the systick mechanism */
-	arm_cm_systick_init();
+	/* set systick to medium priority */
+	NVIC_SetPriority(SysTick_IRQn, arm_cm_medium_priority());
 }
 
 void arch_init(void)