posix: implement `pthread_attr_getscope()`

[ycsin]

This is part of the See POSIX Roadmap for LTSv3 (RFC #51211).

pthread_attr_getscope() is required as part of _POSIX_THREAD_PRIORITY_SCHEDULING Option Group.

For more information, please refer to https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_attr_getscope.html