gh-117439: Make refleak checking thread-safe without the GIL (#117469)

This keeps track of the per-thread total reference count operations in
PyThreadState in the free-threaded builds. The count is merged into the
interpreter's total when the thread exits.

Include/internal/pycore_tstate.h

@@ -38,6 +38,10 @@ typedef struct _PyThreadStateImpl {
     struct _brc_thread_state brc;
 #endif
 
+#if defined(Py_REF_DEBUG) && defined(Py_GIL_DISABLED)
+    Py_ssize_t reftotal;  // this thread's total refcount operations
+#endif
+
 } _PyThreadStateImpl;