To get the boundary cases correct, you need a special case for lcm(0, 0), which should be 0.

Did you have any particular use-cases in mind for this?  It may make sense to allow multiple arguments:  e.g., lcm(4, 5, 6) -> 60.

Overall, I'm -0 on this addition: I don't think it comes up often enough to make it worth adding, and when it does come up it's easy to create the lcm from the gcd (just as your patch does).