riscv: support sleeping in QEMU

QEMU doesn't support the RTC peripheral yet so work around it for now.

This makes the following command work:

    tinygo run -target=hifive1-qemu ./testdata/coroutines.go

src/runtime/runtime_fe310_baremetal.go

@@ -4,6 +4,7 @@ package runtime
 
 import (
 	"device/riscv"
+	"device/sifive"
 )
 
 func abort() {
@@ -12,3 +13,27 @@ func abort() {
 		riscv.Asm("wfi")
 	}
 }
+
+func ticks() timeUnit {
+	// Combining the low bits and the high bits yields a time span of over 270
+	// years without counter rollover.
+	highBits := sifive.RTC.RTCHI.Get()
+	for {
+		lowBits := sifive.RTC.RTCLO.Get()
+		newHighBits := sifive.RTC.RTCHI.Get()
+		if newHighBits == highBits {
+			// High bits stayed the same.
+			println("bits:", highBits, lowBits)
+			return timeUnit(lowBits) | (timeUnit(highBits) << 32)
+		}
+		// Retry, because there was a rollover in the low bits (happening every
+		// 1.5 days).
+		highBits = newHighBits
+	}
+}
+
+func sleepTicks(d timeUnit) {
+	target := ticks() + d
+	for ticks() < target {
+	}
+}