gh-112529: Use GC heaps for GC allocations in free-threaded builds (gh-114157)

* gh-112529: Use GC heaps for GC allocations in free-threaded builds

The free-threaded build's garbage collector implementation will need to
find GC objects by traversing mimalloc heaps. This hooks up the
allocation calls with the correct heaps by using a thread-local
"current_obj_heap" variable.

* Refactor out setting heap based on type