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_qemu.go

@@ -10,7 +10,19 @@ import (
 // Special memory-mapped device to exit tests, created by SiFive.
 var testExit = (*volatile.Register32)(unsafe.Pointer(uintptr(0x100000)))
 
+var timestamp timeUnit
+
 func abort() {
 	// Signal a successful exit.
 	testExit.Set(0x5555)
 }
+
+func ticks() timeUnit {
+	return timestamp
+}
+
+func sleepTicks(d timeUnit) {
+	// Note: QEMU doesn't seem to support the RTC peripheral at the time of
+	// writing so just simulate sleeping here.
+	timestamp += d
+}