os: fix windows and plan9 builds

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12519046

src/pkg/os/file.go

@@ -238,3 +238,6 @@ func Open(name string) (file *File, err error) {
 func Create(name string) (file *File, err error) {
 	return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
 }
+
+// lstat is overridden in tests.
+var lstat = Lstat

src/pkg/os/file_unix.go

@@ -149,9 +149,6 @@ func Lstat(name string) (fi FileInfo, err error) {
 	return fileInfoFromStat(&stat, name), nil
 }
 
-// lstat is overridden in tests.
-var lstat = Lstat
-
 func (f *File) readdir(n int) (fi []FileInfo, err error) {
 	dirname := f.name
 	if dirname == "" {