powerpc/ptrace: move register viewing functions out of ptrace.c

Create a dedicated ptrace-view.c file.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bfd8c3ed57c9057e4a5d3816737b5ee98c6f7e43.1582848567.git.christophe.leroy@c-s.fr