gh-113055: Use pointer for interp->obmalloc state (gh-113412)

For interpreters that share state with the main interpreter, this points
to the same static memory structure.  For interpreters with their own
obmalloc state, it is heap allocated.  Add free_obmalloc_arenas() which
will free the obmalloc arenas and radix tree structures for interpreters
with their own obmalloc state.

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>