powerpc: move common register copy functions from signal_32.c to signal.c

These functions are required for 64bit as well.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9fd6d9b7c5e91fab21159fe23534a2f16b4962d3.1584699455.git.msuchanek@suse.de