powerpc/32: Reorder instructions to avoid using CTR in syscall entry

Now that we are using rfi instead of mtmsr to reactivate MMU, it is
possible to reorder instructions and avoid the need to use CTR for
stashing SRR0.

null_syscall on 8xx is reduced by 3 cycles (283 => 280 cycles).

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8fa13a59f73647e058c95fc7e1c7a98f316bd20a.1612796617.git.christophe.leroy@csgroup.eu