os: fix Args setup on Windows

Should fix the Windows build. Untested.

on Windows, args are made by src/os/exec_windows.go, not package runtime.
runtime·goargs has if(Windows) return;

The two init funcs in pkg os were conflicting, with the second
overwriting Args back to an empty slice.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/143540044

Committer: Russ Cox <rsc@golang.org>