add rp2040, pico

adds preliminary support (just enough to run blinky1) for the Raspberry Pi Pico board along with the rp2040 mcu.

src/runtime/runtime_rp2040.go

@@ -0,0 +1,58 @@
+// +build rp2040
+
+package runtime
+
+import (
+	"device/arm"
+)
+
+// machineTicks is provided by package machine.
+func machineTicks() uint64
+
+type timeUnit uint64
+
+// ticks returns the number of ticks (microseconds) elapsed since power up.
+func ticks() timeUnit {
+	t := machineTicks()
+	return timeUnit(t)
+}
+
+func ticksToNanoseconds(ticks timeUnit) int64 {
+	return int64(ticks) * 1000
+}
+
+func nanosecondsToTicks(ns int64) timeUnit {
+	return timeUnit(ns / 1000)
+}
+
+func sleepTicks(d timeUnit) {
+	if d == 0 {
+		return
+	}
+	sleepUntil := ticks() + d
+	for ticks() < sleepUntil {
+	}
+}
+
+func waitForEvents() {
+	arm.Asm("wfe")
+}
+
+func putchar(c byte) {
+}
+
+// machineInit is provided by package machine.
+func machineInit()
+
+func init() {
+	machineInit()
+}
+
+func postinit() {}
+
+//export Reset_Handler
+func main() {
+	preinit()
+	run()
+	abort()
+}