In the free-threaded build, avoid data races caused by updating type
slots or type flags after the type was initially created.  For those
(typically rare) cases, use the stop-the-world mechanism.  Remove the
use of atomics when reading or writing type flags.  The use of atomics
is not sufficient to avoid races (since flags are sometimes read without
a lock and without atomics) and are no longer required.

To avoid deadlocks while the world is stopped, we need to avoid calling APIs
like _PyObject_HashFast() and _PyDict_GetItemRef_KnownHash().  Collect the
slot updates to be done and then apply them all at once.  This reduces the
amount of code running in the stop-the-world condition.

Now that stop-the-world is used to do the slot update, these tests
are a lot slower in the free-threaded build.  Test with fewer items,
which will still hopefully be enough to find bugs in the specializer.

The clearing of Py_TPFLAGS_HAVE_VECTORCALL must be done when the world
is stopped too.

Since we stack allocate one chunk, we need to check 'n' to see if there
are actually any updates to make.  It's pretty common that no updates
are actually needed.

If the two mutex form of the critical section is used, need to put the
other mutex into '_cs_mutex'.