MONTGOMERY, Ala. — Japanese automakers Toyota and Mazda on Wednesday announced plans to build a huge, $1.6 billion joint-venture plant in Alabama that will eventually employ about 4,000 people. Several states had competed for the coveted project, which will be able to turn out 300,000 vehicles per year and will produce the Toyota Corolla compact car for North America and a new, small SUV from Mazda.