gh-114271: Make `thread._rlock` thread-safe in free-threaded builds (#115102)

gh-114271: Make `thread._rlock` thread-safe in free-threaded builds (#115102)

The ID of the owning thread (`rlock_owner`) may be accessed by
multiple threads without holding the underlying lock; relaxed
atomics are used in place of the previous loads/stores.

The number of times that the lock has been acquired (`rlock_count`)
is only ever accessed by the thread that holds the lock; we do not
need to use atomics to access it.