powerpc/32: Use START_EXCEPTION() as much as possible

Everywhere where it is possible, use START_EXCEPTION().

This will help for proper exception init in future patches.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d47c1cc242bbbef8658327503726abdaef9b63ef.1615552867.git.christophe.leroy@csgroup.eu