powerpc/inst: Refactor PPC32 and PPC64 versions

ppc_inst() ppc_inst_prefixed() ppc_inst_swab() can easily be made common
to both PPC32 and PPC64.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d54c63dcac6d190e1cc0d2fe3259d6e621928cdf.1621516826.git.christophe.leroy@csgroup.eu