device/arm: add system timer registers (#654)

* device/arm: add system timer registers
Add SYST registers and bit definitions to device/arm.
Add a setup function.
Add an example that uses it to blink an LED.

src/examples/systick/README.md

@@ -0,0 +1,12 @@
+# TinyGo ARM SysTick example
+
+This example uses the ARM System Timer to blink an LED.  The timer fires
+an interrupt 10 times per second.  The interrupt handler toggles the LED on
+and off.
+
+Many ARM-based chips have this timer feature.  If you run the example and the
+LED blinks, then you have one.
+
+The System Timer runs from a cycle counter.  The more cycles, the slower the
+LED will blink.  This counter is 24 bits wide, which places an upper bound on
+the number of cycles, and the slowness of the blinking.