Saudi Arabia is slashing its U.S. oil exports to a nearly three-decade low for this time of the year, intensifying its efforts to reduce a global supply glut that has been pummeling crude prices. State-owned Saudi Arabian Oil Co. expects sales to the U.S. will drop below 1 million barrels a day in June, then slide to about 850,000 barrels a day in July, according to people familiar with the matter. The July figure would be its lowest…