* European stocks extend gains; weak euro helps* Weak German data fuels ECB QE expectations, lift bonds* Dollar index near 4-year high on solid U.S. payroll gains* Brazil&aposs Rousseff faces run-off electionBy Emelia Sithole-MatariseLONDON, Oct 6 (Reuters) - European equities advanced while the dollar held near four-year highs against a basket of currencies on Monday after stronger-than-expected