add support for darwin arm64

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

Signed-off-by: Tobias Kohlbau <tobias@kohlbau.de>

serial_darwin_64.go

@@ -0,0 +1,56 @@
+// +build darwin
+// +build amd64 arm64
+
+//
+// Copyright 2014-2020 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.
+//
+
+package serial
+
+import "golang.org/x/sys/unix"
+
+// termios manipulation functions
+
+var baudrateMap = map[int]uint64{
+	0:      unix.B9600, // Default to 9600
+	50:     unix.B50,
+	75:     unix.B75,
+	110:    unix.B110,
+	134:    unix.B134,
+	150:    unix.B150,
+	200:    unix.B200,
+	300:    unix.B300,
+	600:    unix.B600,
+	1200:   unix.B1200,
+	1800:   unix.B1800,
+	2400:   unix.B2400,
+	4800:   unix.B4800,
+	9600:   unix.B9600,
+	19200:  unix.B19200,
+	38400:  unix.B38400,
+	57600:  unix.B57600,
+	115200: unix.B115200,
+	230400: unix.B230400,
+}
+
+var databitsMap = map[int]uint64{
+	0: unix.CS8, // Default to 8 bits
+	5: unix.CS5,
+	6: unix.CS6,
+	7: unix.CS7,
+	8: unix.CS8,
+}
+
+const tcCMSPAR uint64 = 0 // may be CMSPAR or PAREXT
+const tcIUCLC uint64 = 0
+
+const tcCCTS_OFLOW uint64 = 0x00010000
+const tcCRTS_IFLOW uint64 = 0x00020000
+
+const tcCRTSCTS uint64 = (tcCCTS_OFLOW | tcCRTS_IFLOW)
+
+func toTermiosSpeedType(speed uint64) uint64 {
+	return speed
+}