os: add Perm stub on Go 1.15

This adds a stub for Perm on Go 1.15, which allows the OS package to compile again.

src/os/file_go_other.go

@@ -1,3 +1,4 @@
+//go:build !go1.16
 // +build !go1.16
 
 package os
@@ -49,3 +50,8 @@ func (m FileMode) IsDir() bool {
 func (m FileMode) IsRegular() bool {
 	return false
 }
+
+// Perm is a stub, always returning 0.
+func (m FileMode) Perm() FileMode {
+	return 0
+}