rb_mutex_synchronize should pass the return value as-is

[eregon]

Found by @jeremyevans through:

• https://github.com/jeremyevans/etc/actions/runs/10012691202/job/27678723077
• jeremyevans/etc@7c2b434
• Release GVL around pw and gr function calls in Etc ruby/etc#40

So in this change, VALUE rb_mutex_synchronize(VALUE mutex, VALUE (*func)(VALUE arg), VALUE arg); is used.
But that function is poorly typed, every VALUE in there should be void* instead, except for VALUE mutex.
That is, both arg and the return value of func can be arbitrary things, not necessary Ruby values/handles.

We already pass arg as-is, but we unwrap+wrap the return value, which I highly suspect is the cause of that error in CI.
So we should remove the unwrap+wrap for rb_mutex_synchronize.

[andrykonchin]

Fixed in 0654e66