selftests/powerpc: Add prefixed loads/stores to alignment_handler test

Extend the alignment handler selftest to exercise prefixed load store
instructions. Add tests for prefixed VSX, floating point and integer
instructions.

Skip prefix tests if ISA version does not support prefixed instructions.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Tested-by: Alistair Popple <alistair@popple.id.au>
[mpe: Fixup PPC_FEATURE2_ARCH_3_1 naming as noted by Alistair]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200520021103.19798-2-jniethe5@gmail.com