powerpc/32: Always save non volatile GPRs at syscall entry

In preparation for porting syscall entry/exit to C, inconditionally
save non volatile general purpose registers.

Commit 965dd3ad ("powerpc/64/syscall: Remove non-volatile GPR save
optimisation") provides detailed explanation.

This increases the number of cycles by 24 cycles on 8xx with
null_syscall benchmark (280 => 304 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/21c08162b83655195fe9ead78ff2cfd28508d023.1612796617.git.christophe.leroy@csgroup.eu