powerpc/sstep: Add tests for Prefixed Add Immediate

Use the existing support for testing compute type instructions to test
Prefixed Add Immediate (paddi). The R bit of the paddi instruction
controls whether current instruction address is used. Add test cases
for when R=1 and for R=0. paddi has a 34 bit immediate field formed by
concatenating si0 and si1. Add tests for the extreme values of this
field.

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

Some of these test cases were added by Balamuruhan S.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: Fix conflicts with ppc-opcode.h changes, squash in .balign]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200525025923.19843-5-jniethe5@gmail.com