powerpc/sstep: Add tests for prefixed floating-point load/stores

Add tests for the prefixed versions of the floating-point load/stores
that are currently tested. This includes the following instructions:
  * Prefixed Load Floating-Point Single (plfs)
  * Prefixed Load Floating-Point Double (plfd)
  * Prefixed Store Floating-Point Single (pstfs)
  * Prefixed Store Floating-Point Double (pstfd)

Skip the new tests if ISA v3.10 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/20200525025923.19843-2-jniethe5@gmail.com