Unit testing for def/ module.

Added unit tests to test code in def/ module.

vendor/golang.org/x/sys/cpu/byteorder.go

@@ -0,0 +1,30 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+import (
+	"encoding/binary"
+	"runtime"
+)
+
+// hostByteOrder returns binary.LittleEndian on little-endian machines and
+// binary.BigEndian on big-endian machines.
+func hostByteOrder() binary.ByteOrder {
+	switch runtime.GOARCH {
+	case "386", "amd64", "amd64p32",
+		"arm", "arm64",
+		"mipsle", "mips64le", "mips64p32le",
+		"ppc64le",
+		"riscv", "riscv64":
+		return binary.LittleEndian
+	case "armbe", "arm64be",
+		"mips", "mips64", "mips64p32",
+		"ppc", "ppc64",
+		"s390", "s390x",
+		"sparc", "sparc64":
+		return binary.BigEndian
+	}
+	panic("unknown architecture")
+}