They have other ways to independently halt. Obviously any core can carry out a calculation, like calculating some arbitrarily chosen recursive primitive function, while the other cores do something completely independent, or execute a halting instruction.
What about a Niagra T1? All eight cores share a single floating point execution unit. They literally can't be clocked separately. And it's not like they're eight "cores" that are really eight SMT threads that are veneers of the same core. They are completely separate cores except the floating point unit.
