rp2040: replace sleep 'busy loop' with timer alarm

src/runtime/runtime_rp2040.go

@@ -12,6 +12,9 @@ import (
 // machineTicks is provided by package machine.
 func machineTicks() uint64
 
+// machineLightSleep is provided by package machine.
+func machineLightSleep(uint64)
+
 type timeUnit uint64
 
 // ticks returns the number of ticks (microseconds) elapsed since power up.
@@ -32,6 +35,16 @@ func sleepTicks(d timeUnit) {
 	if d == 0 {
 		return
 	}
+
+	if hasScheduler {
+		// With scheduler, sleepTicks may return early if an interrupt or
+		// event fires - so scheduler can schedule any go routines now
+		// eligible to run
+		machineLightSleep(uint64(d))
+		return
+	}
+
+	// Busy loop
 	sleepUntil := ticks() + d
 	for ticks() < sleepUntil {
 	}