powerpc/signal32: Misc changes to make handle_[rt_]_signal32() more similar

Miscellaneous changes to clean and make handle_signal32() and
handle_rt_signal32() even more similar.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/df0bc8c3b8fa96390c46f611df79b2a94ac21844.1597770847.git.christophe.leroy@csgroup.eu