Toyota said on Friday that it was taking a 5 percent stake in Mazda, another Japanese automaker, adding that the companies would jointly build a new assembly plant in the United States and pool resources on new technologies.The factory's location has not been decided, but Toyota and Mazda said they hoped the first vehicles would roll off its production lines in 2021. The plant is expected to cost $1.6 billion and will employ about 4,000 workers, they said.