Use GetFileAttributesW() to see if a file exists

_wstat64() does not work on drives, so e.g. for C:\ it would always
return that it doesn't exist.

Ideally, in the future, all calls to _wstat64() would be replaced with
native calls that return the same information, to avoid any
inconsistencies. At that point, the Amiga re-implementation of of_stat()
could be deprecated as well.