cpu: add test for IsBigEndian

For golang/go#57237

Change-Id: I11d1c954f942ebd836c4deb9c710c40778dc5599
Reviewed-on: https://go-review.googlesource.com/c/sys/+/494858
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

cpu/endian_test.go

@@ -0,0 +1,21 @@
+// Copyright 2023 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_test
+
+import (
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/cpu"
+)
+
+func TestIsBigEndian(t *testing.T) {
+	b := uint16(0xff00)
+	want := *(*byte)(unsafe.Pointer(&b)) == 0xff
+	if cpu.IsBigEndian != want {
+		t.Errorf("IsBigEndian = %t, want %t",
+			cpu.IsBigEndian, want)
+	}
+}