runtime: move KeepAlive/SetFinalizer to common code

We don't support these yet so let's just put them in a central location.
Once these functions are supported we can think about how to structure
the code again.

src/runtime/runtime.go

@@ -87,3 +87,11 @@ func LockOSThread() {
 // Stub for now
 func UnlockOSThread() {
 }
+
+func KeepAlive(x interface{}) {
+	// Unimplemented. Only required with SetFinalizer().
+}
+
+func SetFinalizer(obj interface{}, finalizer interface{}) {
+	// Unimplemented.
+}