Removed obsolete +build directives in favor of modern go:build

serial_darwin_64.go

@@ -1,12 +1,11 @@
-// +build darwin
-// +build amd64 arm64
-
 //
 // Copyright 2014-2021 Cristian Maglie. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 //
 
+//go:build darwin && (amd64 || arm64)
+
 package serial
 
 import "golang.org/x/sys/unix"