gh-129668: Fix thread-safety of MemoryError freelist in free threaded build (gh-129704)

The MemoryError freelist was not thread-safe in the free threaded build.
Use a mutex to protect accesses to the freelist. Unlike other freelists,
the MemoryError freelist is not performance sensitive.

Include/internal/pycore_exceptions.h

@@ -24,6 +24,9 @@ struct _Py_exc_state {
     PyObject *errnomap;
     PyBaseExceptionObject *memerrors_freelist;
     int memerrors_numfree;
+#ifdef Py_GIL_DISABLED
+    PyMutex memerrors_lock;
+#endif
     // The ExceptionGroup type
     PyObject *PyExc_ExceptionGroup;
 };