runtime: implement internal/godebug.setUpdate as a stub

This function provides a mechanism to watch for changes to the GODEBUG
environment variable. For now, we'll not implement it. It might be
useful in the future, when it can always be added.

src/runtime/runtime.go

@@ -95,3 +95,9 @@ func KeepAlive(x interface{}) {
 func SetFinalizer(obj interface{}, finalizer interface{}) {
 	// Unimplemented.
 }
+
+//go:linkname godebug_setUpdate internal/godebug.setUpdate
+func godebug_setUpdate(update func(string, string)) {
+	// Unimplemented. The 'update' function needs to be called whenever the
+	// GODEBUG environment variable changes (for example, via os.Setenv).
+}