powerpc/40x: Reorder a few instructions in critical exception prolog

In order to ease preparation for CONFIG_VMAP_STACK, reorder
a few instruction, especially save r1 into stack frame earlier.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c895ecf958c86d1736bdd2ff6f36626b55f35fd2.1615552866.git.christophe.leroy@csgroup.eu