powerpc/sstep: Add tests for prefixed integer load/stores

Add tests for the prefixed versions of the integer load/stores that
are currently tested. This includes the following instructions:
  * Prefixed Load Doubleword (pld)
  * Prefixed Load Word and Zero (plwz)
  * Prefixed Store Doubleword (pstd)

Skip the new tests if ISA v3.1 is unsupported.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: Fix conflicts with ppc-opcode.h changes]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200525025923.19843-1-jniethe5@gmail.com