A lawyer for Andrija Artukovic, the man accused of masterminding the slaughter of 770,000 Jews, Serbs and Gypsies during World War II, said his client apparently has suffered a stroke at the Long Beach Naval Hospital. A hospital spokesman said it was not serious. Attorney Ronald Bonaparte said doctors at the hospital, where the 85-year-old Artukovic is being held without bail pending an extradition hearing in a Los Angeles federal court next week, reported that the defendant had "spasms of vomiting and was not in good shape. . . . My interpretation of what the doctor said was that he seemed to suffer some stroke or disorder. . . ." Hospital spokesman Cmdr. Howard Sixsmith said Artukovic suffered only nausea and vomiting, and tests turned out to "be within normal parameters." Artukovic was well and back to normal, Sixsmith said.