add support for darwin arm64

Support arm64 which is used by the new lineup of computers
from apple.

Signed-off-by: Tobias Kohlbau <[email protected]>

Author: tobiaskohlbau

serial_darwin_amd64.go renamed to serial_darwin_64.go

@@ -1,3 +1,6 @@
+// +build darwin
+// +build amd64 arm64
+
 //
 // Copyright 2014-2020 Cristian Maglie. All rights reserved.
 // Use of this source code is governed by a BSD-style