By Alexander Marrow MOSCOW (Reuters) -The rouble weakened on Friday, giving up huge gains that had seen it soar to an eight-year high against the euro, as investors slashed foreign currency holdings and President Vladimir Putin signed treaties annexing four Ukrainian regions. Western governments and Kyiv say the move breaches international law. By 1355 GMT, the rouble had lost 1.6% to trade at 56.48 versus the euro, swinging back from its strongest mark since early October 2014 of 50.7250. It was 1.7% weaker against the dollar at 58.20, giving up massive gains that had earlier seen it touch it…