A jury in Los Angeles has assessed punitive damages of more than $760 million against five oil and chemical companies for injuries suffered by workers building the Lockheed Corp., according to lawyers involved in the case. The jury verdict, handed up in the Los Angeles County Superior Court, followed a finding that the companies had not provided adequate warning about the toxic effects of chemicals such as the solvents that were used in the manufacture of the radar-evading aircraft in the 1960s,1970s and 1980s.