powerpc/sstep: Check instruction validity against ISA version before emulation

We currently unconditionally try to emulate newer instructions on older
Power versions that could cause issues. Gate it.

Fixes: 350779a2 ("powerpc: Handle most loads and stores in instruction emulation code")
Signed-off-by: Ananth N Mavinakayanahalli <ananth@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/161157995977.64773.13794501093457185080.stgit@thinktux.local