gh-112532: Isolate abandoned segments by interpreter (#113717)

* gh-112532: Isolate abandoned segments by interpreter

Mimalloc segments are data structures that contain memory allocations along
with metadata. Each segment is "owned" by a thread. When a thread exits,
it abandons its segments to a global pool to be later reclaimed by other
threads. This changes the pool to be per-interpreter instead of process-wide.

This will be important for when we use mimalloc to find GC objects in the
`--disable-gil` builds. We want heaps to only store Python objects from a
single interpreter. Absent this change, the abandoning and reclaiming process
could break this isolation.

* Add missing '&_mi_abandoned_default' to 'tld_empty'

Python/pystate.c

@@ -2533,6 +2533,11 @@ tstate_mimalloc_bind(PyThreadState *tstate)
     mi_tld_t *tld = &mts->tld;
     _mi_tld_init(tld, &mts->heaps[_Py_MIMALLOC_HEAP_MEM]);
 
+    // Exiting threads push any remaining in-use segments to the abandoned
+    // pool to be re-claimed later by other threads. We use per-interpreter
+    // pools to keep Python objects from different interpreters separate.
+    tld->segments.abandoned = &tstate->interp->mimalloc.abandoned_pool;
+
     // Initialize each heap
     for (Py_ssize_t i = 0; i < _Py_MIMALLOC_HEAP_COUNT; i++) {
         _mi_heap_init_ex(&mts->heaps[i], tld, _mi_arena_id_none());