gh-127266: avoid data races when updating type slots (gh-131174)

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.