gh-112532: Fix memory block count for free-threaded build (gh-113995)

This fixes `_PyInterpreterState_GetAllocatedBlocks()` and
`_Py_GetGlobalAllocatedBlocks()` in the free-threaded builds. The
gh-113263 change that introduced multiple mimalloc heaps per-thread
broke the logic for counting the number of allocated blocks. For subtle
reasons, this led to reported reference count leaks in the refleaks
buildbots.