ARM: extend the SysTick handler to also call dda_clock().

As dda_clock() is potantially too slow for high step rates, we
call it with a secondary interrupt with slightly slower priority.
This makes sure the slow part is ignored on high system load,
still reasonably synchonized with the clock tick.

Test: steppers should move and accelerate now.

Current binary size:

    SIZES          ARM...     lpc1114
    FLASH  :  7756 bytes          24%
    RAM    :   960 bytes          24%
    EEPROM :     0 bytes           0%

timer-arm.c

@@ -20,6 +20,13 @@
 
   For the system clock, we use SysTickTimer. This timer is made for exactly
   such purposes.
+
+  Other than AVRs, Cortex-M doesn't allow reentry of interupts. To deal with
+  this we use another interrupt, PendSV, with slightly lower priority and
+  without an interrupt source. The possibly lengthy operation in dda_clock()
+  goes into there and at the end of the SysTick interrupt we simply set PendSV
+  pending. This way PendSV is executed right after SysTick or, if PendSV is
+  already running, ignored.
 */
 void timer_init() {
 
@@ -48,6 +55,11 @@ void timer_init() {
                  | SysTick_CTRL_TICKINT_Msk       // Enable interrupt.
                  | SysTick_CTRL_CLKSOURCE_Msk;    // Run at full CPU clock.
 
+  /**
+    Initialise PendSV for dda_clock().
+  */
+  NVIC_SetPriority(PendSV_IRQn, 1);               // Almost highest priority.
+
   /**
     Initialise the stepper timer. On ARM we have the comfort of hardware
     32-bit timers, so we don't have to artifically extend the timer to this
@@ -79,9 +91,17 @@ void timer_init() {
 void SysTick_Handler(void) {
 
   clock_tick();
-  #ifndef __ARMEL_NOTYET__
+
+  SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;             // Trigger PendSV_Handler().
+}
+
+/** System clock interrupt, slow part.
+
+  Here we do potentially lengthy calculations. Must have the same name as in
+  cmsis-startup_lpc11xx.s
+*/
+void PendSV_Handler(void) {
   dda_clock();
-  #endif /* __ARMEL_NOTYET__ */
 }
 
 /** Step interrupt.