gh-101659: Isolate "obmalloc" State to Each Interpreter (gh-101660)

This is strictly about moving the "obmalloc" runtime state from
`_PyRuntimeState` to `PyInterpreterState`.  Doing so improves isolation
between interpreters, specifically most of the memory (incl. objects)
allocated for each interpreter's use.  This is important for a
per-interpreter GIL, but such isolation is valuable even without it.

FWIW, a per-interpreter obmalloc is the proverbial
canary-in-the-coalmine when it comes to the isolation of objects between
interpreters.  Any object that leaks (unintentionally) to another
interpreter is highly likely to cause a crash (on debug builds at
least).  That's a useful thing to know, relative to interpreter
isolation.

Objects/object.c

@@ -145,7 +145,7 @@ _PyDebug_PrintTotalRefs(void) {
     _PyRuntimeState *runtime = &_PyRuntime;
     fprintf(stderr,
             "[%zd refs, %zd blocks]\n",
-            get_global_reftotal(runtime), _Py_GetAllocatedBlocks());
+            get_global_reftotal(runtime), _Py_GetGlobalAllocatedBlocks());
     /* It may be helpful to also print the "legacy" reftotal separately.
        Likewise for the total for each interpreter. */
 }