NOGALES, MEXICO—After nearly 20 years on the run, Grammy-winning singer-songwriter Christopher Cross finally reached the Mexican border Monday. "I had such a long way to go,"said Cross, who fled south after gunning down 10 in 1980, "but I've finally made it to the border of Mexico." Doctors, who described Cross' body as "weak," said much sleep would be necessary to restore the health of the fugitive adult-contemporary vocalist.