gh-117657: Fix data races in the method cache in free-threaded builds (#117954)

Fix data races in the method cache in free-threaded builds

These are technically data races, but I think they're benign (to
the extent that that is actually possible). We update cache entries
non-atomically but read them atomically from another thread, and there's
nothing that establishes a happens-before relationship between the
reads and writes that I can see.