GLOBAL MARKETS-Stock markets fall; U.S. dollar at multi-month low

* Equities move lower in U.S. and Europe* Dollar drops to 6-1/2-month low versus currency basket* Tension between Russia, Ukraine weighs on markets(Updates prices to U.S. close)By Ryan VlastelicaNEW YORK, May 6 (Reuters) - Stock markets around the world fell on Tuesday, with U.S. shares extending declines in afternoon trading as persistent worries over the instability in Ukraine pushed investors t