The signal handler in the alignment handler self test has the ability
to jump over the instruction that triggered the signal. It does this
by incrementing the PT_NIP in the user context by 4. If it were a
prefixed instruction this will mean that the suffix is then executed
which is incorrect. Instead check if the major opcode indicates a
prefixed instruction (e.g. it is 1) and if so increment PT_NIP by 8.

If ISA v3.1 is not available treat it as a word instruction even if
the major opcode is 1.

Fixes: 620a6473 ("selftests/powerpc: Add prefixed loads/stores to alignment_handler test")
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: Fix 32-bit build, rename haveprefixes to prefixes_enabled]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200824131231.14008-1-jniethe5@gmail.com