Michael Dell’s return to the public markets after a protracted absence has helped make him $12 billion wealthier. The man who became famous selling computers more cheaply over a newish thing called the internet retreated from Wall Street’s glare in 2013, delisting a company in which he held a stake of just $3.8 billion as sales of personal computers sagged. He rebuilt the company, turning maker of inexpensive PCs into a diversified tech powerhouse that sells everything from servers to security software.