Massachusetts Institute of Technology is taking a particularly ambitious step, creating a new college backed by a planned investment of $1 billion. Two-thirds of the funds have already been raised, M.I.T. said, in announcing the initiative on Monday. The linchpin gift of $350 million came from Stephen A. Schwarzman, chief executive of the Blackstone Group, the big private equity firm. The college, called the M.I.T. Stephen A. Schwarzman College of Computing, will create 50 new faculty positions and many more fellowships for graduate students.