RFR: 8232107: support ThreadPriorityPolicy flag on AIX

[Baesken, Matthias]

Hi Christoph/David, thanks for the reviews !

Best regards, Matthias

> Hi Matthias, David,
> 
> first of all, +1 from my side.
> 
> I think it's beneficial to make AIX behave most similar to Linux at this place.
> Whether the whole feature should be terminally deprecated and removed is
> a different discussion. Just my 5cents ��
> 
> Cheers
> Christoph
>