[testbug][aix] Various test adaption to aix pecularities.

CanGenerateAllClassHook.java:
Needs @requires vm.cds.

FPRegs.java:
The test implements a custom launcher. The launcher starts the
VM in the primordial thread, which is not possible on aix.
Just exclude the test.

WBStackSize.java:
The sizes of stacks of threads returned by pthread differ from
the requested stack size. Thus stack sizes vary w.r.t. the
stack size given in flags. Relax the test on aix.

TestInheritFD.java:
On Aix lsof requires root privileges. Exclude the test.