Rename built-in SGN() macro to SIGN()

This matches the name of the GDScript function (except it's uppercase
here).

core/math/vector2.cpp

@@ -91,7 +91,7 @@ real_t Vector2::cross(const Vector2 &p_other) const {
 }
 
 Vector2 Vector2::sign() const {
-	return Vector2(SGN(x), SGN(y));
+	return Vector2(SIGN(x), SIGN(y));
 }
 
 Vector2 Vector2::floor() const {