* Saudi petrochemicals helped by strong U.S. eco**mic data * But most banks pull back after sharp gains * Dubai's Arabtec jumps on hopes Aabar will increase stake * Arabtec ex-CEO says wants over 5 dirhams per share * Gulf Finance House ends well off high after loan deal By Olzhas Auyezov DUBAI, Aug 27 (Reuters) - Expectations of strong global demand for petrochemicals supported the uptrend in Saudi Arabia's stock market on Wednesday, lifting the bourse to a fresh six-year high, while Dubai builder Arabtec jumped on hopes that a key shareholder would increase its stake. ...