gh-117783: Immortalize objects that use deferred reference counting (#118112)

Deferred reference counting is not fully implemented yet. As a temporary
measure, we immortalize objects that would use deferred reference
counting to avoid multi-threaded scaling bottlenecks.

This is only performed in the free-threaded build once the first
non-main thread is started. Additionally, some tests, including refleak
tests, suppress this behavior.

Python/pystate.c

@@ -1568,6 +1568,17 @@ new_threadstate(PyInterpreterState *interp, int whence)
         // Must be called with lock unlocked to avoid re-entrancy deadlock.
         PyMem_RawFree(new_tstate);
     }
+    else {
+#ifdef Py_GIL_DISABLED
+        if (interp->gc.immortalize.enable_on_thread_created &&
+            !interp->gc.immortalize.enabled)
+        {
+            // Immortalize objects marked as using deferred reference counting
+            // the first time a non-main thread is created.
+            _PyGC_ImmortalizeDeferredObjects(interp);
+        }
+#endif
+    }
 
 #ifdef Py_GIL_DISABLED
     // Must be called with lock unlocked to avoid lock ordering deadlocks.