go/ssa/interp: add syscall.Sysctl to Darwin.
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://codereview.appspot.com/68000048

d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,webfd/go-zh.tools