Cached boxed numerics are always boxed as int

Discovered in https://github.com/HaxeFoundation/haxe/pull/12410, doubles and int64s have the same small boxed value caching done, but they're put in the same cache as ints and are boxed as ints.
So when a int64 is boxed and it's boxed form is pulled from the cache it will report its type as just int.