Heap size should be added at end of the struct

Co-authored-by: Dino Viehland <dinoviehland@meta.com>
Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>

Author: sergey-miryanov

Include/internal/pycore_interp_structs.h

@@ -228,11 +228,11 @@ struct _gc_runtime_state {
     /* linked lists of container objects */
 #ifndef Py_GIL_DISABLED
     struct gc_generation generations[NUM_GENERATIONS];
-    PyGC_Head *generation0;
 #else
     struct gc_generation young;
     struct gc_generation old[2];
 #endif
+
     /* a permanent generation which won't be collected */
     struct gc_generation permanent_generation;
     struct gc_stats *generation_stats;
@@ -249,6 +249,11 @@ struct _gc_runtime_state {
     /* The number of live objects. */
     Py_ssize_t heap_size;
 
+    /* dummy members to preserve other offsets */
+    Py_ssize_t dummy1; /* was work_to_do */
+    int dummy2; /* was visited_space */
+    int dummy3; /* was phase */
+
     /* This is the number of objects that survived the last full
        collection. It approximates the number of long lived objects
        tracked by the GC.
@@ -274,6 +279,8 @@ struct _gc_runtime_state {
 
     /* Mutex held for gc_should_collect_mem_usage(). */
     PyMutex mutex;
+#else
+    PyGC_Head *generation0;
 #endif
 };