port of fix from 3.5.x for issue #3408 - getprocs64 on aix

This ports a change made to 3.5.x (ecf826e0b150d445ab3c942424291181b099f09f) into master - configure checks if getprocs64 exists and either enables or disables function delaration of getprocs64 in process_aix and a unit test.