The Senate confirmed thousands of nominations blocked by Democratic leader Robert C. Byrd (D-W.Va.) after getting White House assurances that President Reagan will give advance notice of recess appointments. The Senate, by voice vote, approved 76 civilian nominations and more than 5,000 military promotions, some held up since Sept. 9. The nominees included federal judges, sub-Cabinet officials, members of advisory panels, U.S. attorneys and marshals. "We're pleased that Sen. Byrd has seen fit to release his hold on our nominees," White House spokesman Larry Speakes said. "I felt there had been too many recess appointments and too often when they were not of an urgent nature," Byrd explained. He said the White House now would inform him sufficiently in advance.