At the Inverclyde Advice and Employment Rights Center in Greenock, Scotland, two dozen women meet. They recite a litany of medical problems:cancers, birth defects, multiple miscarriages. The women blame the toll on one thing : They say that for years, while they made computer chips at National Semiconductor Corp.'s plant they were barraged with toxic chemicals that spilled, leaked and sometimes exploded from chip-making equipment. Semiconductor manufacturers always have potrayed their industry as clean, free of smokestacks, with no visible pollution and no health risks. That image, however, is being challenged by some occupational-health experts, government scientists and chip workers.