posix: implement `sched_getparam()`

[ycsin]

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

sched_getparam() is required as part of _POSIX_PRIORITY_SCHEDULING Option Group.

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

[golowanow]

related/depends on #67017

[AbhinavMir]

Hi! Is sched_getparam implemented in full? I checked the merged commits and seems like it's currently keeping the compiler happy and returning ENOSYS / -1. Is this a stub to be implemented later? Thanks.