powerpc/signal32: Isolate non-copy actions in save_user_regs() and save_tm_user_regs()

Reorder actions in save_user_regs() and save_tm_user_regs() to
regroup copies together in order to switch to user_access_begin()
logic in a later patch.

Move non-copy actions into new functions called
prepare_save_user_regs() and prepare_save_tm_user_regs().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f6eac65781b4a57220477c8864bca2b57f29a5d5.1597770847.git.christophe.leroy@csgroup.eu