RFR: 8232107: support ThreadPriorityPolicy flag on AIX

[Baesken, Matthias]

Hello, please review the following small change .
It adds  support  for the   ThreadPriorityPolicy flag on AIX   (and brings the implementation in os_aix  closer  to  what we have in os_linux ).

While  ThreadPriorityPolicy   is an "old"   flag, we have some customers using it  so it would be nice to support it  on all UNIX platforms .


Bug / webrev :

https://bugs.openjdk.java.net/browse/JDK-8232107

http://cr.openjdk.java.net/~mbaesken/webrevs/8232107.0/


Best regards, Matthias