powerpc/32: transfer can avoid saving r4/r5 over trace call

Now that handlers get all registers from pt_regs, r4 and r5 are no
longer live here and may be clobbered.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-11-npiggin@gmail.com