selftests/powerpc: Add more version checks to alignment_handler test

The alignment_handler is documented to only work on Power8/Power9, but
we can make it run on older CPUs by guarding more of the tests with
feature checks.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>