make mkWeak#,... levity-polymorphic (#17817)

I've taken some of the work from !5218 (closed), and used it for the easiest task, which is simply changing some of the primops to use levity polymorphism (instead of full representation polymorphism), such as mkWeak#. This should be a building block for upcoming refactors to make Array#, as well as other primops, levity-polymorphic.