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

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+# 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.

src/examples/systick/systick.go

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+package main
+
+import (
+	"device/arm"
+	"machine"
+)
+
+func main() {
+	machine.LED.Configure(machine.PinConfig{Mode: machine.PinOutput})
+
+	// timer fires 10 times per second
+	arm.SetupSystemTimer(machine.CPU_FREQUENCY / 10)
+
+	for {
+	}
+}
+
+var led_state bool
+
+//go:export SysTick_Handler
+func timer_isr() {
+	if led_state {
+		machine.LED.Low()
+	} else {
+		machine.LED.High()
+	}
+	led_state = !led_state
+}