unix: add Mremap for netbsd

Fixes golang/go#60409

Change-Id: I2d872a1a6fb63c27ab3753deff370e1c2f752bdd
Reviewed-on: https://go-review.googlesource.com/c/sys/+/508397
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/export_mremap_test.go

@@ -0,0 +1,13 @@
+// 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.
+
+//go:build linux || netbsd
+// +build linux netbsd
+
+package unix
+
+const (
+	MremapFixed   = mremapFixed
+	MremapMaymove = mremapMaymove
+)