GLOBAL MARKETS-Equities slump, led by energy as oil hits 5-1/2-year lows

* Oil prices hit 5-1/2-year lows* Commodity-linked shares lead stock declines* Euro hits near 9-year low vs dollar on "Grexit" fears* U.S. long-dated yields hit multi-year lows(Updates to open of U.S. markets; changes byline; dateline previously LONDON)By Sam ForgioneNEW YORK, Jan 5 (Reuters) - Equity markets worldwide tumbled on Monday, led by commodity-linked shares as oil prices fell to 5-1/2-y