runtime: add runtime.nanotime

This function returns the current timestamp, or 0 at compile time.

runtime.nanotime is used at package initialization by the time package
starting with Go 1.12.

src/runtime/runtime.go

@@ -84,9 +84,13 @@ func sleep(d int64) {
 	sleepTicks(timeUnit(d / tickMicros))
 }
 
+func nanotime() int64 {
+	return int64(ticks()) * tickMicros
+}
+
 //go:linkname now time.now
 func now() (sec int64, nsec int32, mono int64) {
-	mono = int64(ticks()) * tickMicros
+	mono = nanotime()
 	sec = mono / (1000 * 1000 * 1000)
 	nsec = int32(mono - sec*(1000*1000*1000))
 	return