go/ssa/interp: add syscall.Sysctl to Darwin.

This fixes a recent test breakge now that os.init calls it
(on Darwin).

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/68000048

go/ssa/interp/external_darwin.go

@@ -0,0 +1,18 @@
+// Copyright 2014 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
+
+package interp
+
+import "syscall"
+
+func init() {
+	externals["syscall.Sysctl"] = ext۰syscall۰Sysctl
+}
+
+func ext۰syscall۰Sysctl(fr *frame, args []value) value {
+	r, err := syscall.Sysctl(args[0].(string))
+	return tuple{r, wrapError(err)}
+}