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://golang.org/cl/143540044

src/os/proc.go

@@ -6,12 +6,19 @@
 
 package os
 
-import "syscall"
+import (
+	"runtime"
+	"syscall"
+)
 
 // Args hold the command-line arguments, starting with the program name.
 var Args []string
 
 func init() {
+	if runtime.GOOS == "windows" {
+		// Initialized in exec_windows.go.
+		return
+	}
 	Args = runtime_args()
 }