runtime/hifive1: use CLINT peripheral for timekeeping

The CLINT is implemented both on the fe310-g002 chip and in the sifive_e
QEMU machine type. Therefore, use that peripheral for consistency.

The only difference is the clock speed, which runs at 10MHz in QEMU for
some reason instead of 32.768kHz as on the physical HiFive1 boards.

src/runtime/runtime_fe310.go

@@ -14,8 +14,6 @@ import (
 
 type timeUnit int64
 
-const tickMicros = 32768 // RTC runs at 32.768kHz
-
 //go:extern _sbss
 var _sbss unsafe.Pointer
 
@@ -82,3 +80,26 @@ func putchar(c byte) {
 }
 
 const asyncScheduler = false
+
+func ticks() timeUnit {
+	// Combining the low bits and the high bits yields a time span of over 270
+	// years without counter rollover.
+	highBits := sifive.CLINT.MTIMEH.Get()
+	for {
+		lowBits := sifive.CLINT.MTIME.Get()
+		newHighBits := sifive.CLINT.MTIMEH.Get()
+		if newHighBits == highBits {
+			// High bits stayed the same.
+			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 {
+	}
+}