x/net/route: use libc calls on Darwin

Starting with 1.12, we must use syscall versions
of sysctl instead of the raw syscall.

An identical CL went into the source copy at golang.org/x/net/route.
This is just a cherry pick of that CL.
(CL: https://go-review.googlesource.com/c/net/+/148597)

Change-Id: I6286ab3e49f82512491afb5bcf349e89ab5645ab
Reviewed-on: https://go-review.googlesource.com/c/149637Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/vendor/golang_org/x/net/route/empty.s

+// Copyright 2018 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.
+
+// +build darwin,go1.12
+
+// This exists solely so we can linkname in symbols from syscall.

src/vendor/golang_org/x/net/route/syscall.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd netbsd openbsd
+// +build dragonfly freebsd netbsd openbsd
 
 package route
 

src/vendor/golang_org/x/net/route/syscall_go1_11_darwin.go

+// Copyright 2018 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.
+
+// +build !go1.12
+
+package route
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var zero uintptr
+
+func sysctl(mib []int32, old *byte, oldlen *uintptr, new *byte, newlen uintptr) error {
+	var p unsafe.Pointer
+	if len(mib) > 0 {
+		p = unsafe.Pointer(&mib[0])
+	} else {
+		p = unsafe.Pointer(&zero)
+	}
+	_, _, errno := syscall.Syscall6(syscall.SYS___SYSCTL, uintptr(p), uintptr(len(mib)), uintptr(unsafe.Pointer(old)), uintptr(unsafe.Pointer(oldlen)), uintptr(unsafe.Pointer(new)), newlen)
+	if errno != 0 {
+		return error(errno)
+	}
+	return nil
+}

src/vendor/golang_org/x/net/route/syscall_go1_12_darwin.go

+// Copyright 2018 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.
+
+// +build go1.12
+
+package route
+
+import _ "unsafe" // for linkname
+
+//go:linkname sysctl syscall.sysctl
+func sysctl(mib []int32, old *byte, oldlen *uintptr, new *byte, newlen uintptr) error