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.

interp/frame.go

@@ -310,6 +310,8 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 			case callee.Name() == "runtime.makeInterface":
 				uintptrType := callee.Type().Context().IntType(fr.TargetData.PointerSize() * 8)
 				fr.locals[inst] = &LocalValue{fr.Eval, llvm.ConstPtrToInt(inst.Operand(0), uintptrType)}
+			case callee.Name() == "runtime.nanotime":
+				fr.locals[inst] = &LocalValue{fr.Eval, llvm.ConstInt(fr.Mod.Context().Int64Type(), 0, false)}
 			case strings.HasPrefix(callee.Name(), "runtime.print") || callee.Name() == "runtime._panic":
 				// This are all print instructions, which necessarily have side
 				// effects but no results.

interp/scan.go

@@ -28,6 +28,9 @@ func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
 	case "runtime.alloc":
 		// Cannot be scanned but can be interpreted.
 		return &sideEffectResult{severity: sideEffectNone}
+	case "runtime.nanotime":
+		// Fixed value at compile time.
+		return &sideEffectResult{severity: sideEffectNone}
 	case "runtime._panic":
 		return &sideEffectResult{severity: sideEffectLimited}
 	}

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