selftests/powerpc: Add test for VPHN

The goal is to verify vphn_unpack_associativity() parses VPHN numbers
correctly. We feed it with a variety of input values and compare with
expected results.

PAPR+ does not say much about VPHN parsing: I came up with a list of
tests that check many simple cases and some corner ones. I wouldn't
dare to say the list is exhaustive though.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
[mpe: Rework harness logic, rename to test-vphn, add -m64]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>