powerpc/32: Refactor booke critical registers saving

Refactor booke critical registers saving into a few macros
and move it into the exception prolog directly.

Keep the dedicated transfert_to_handler entry point for the
moment allthough they are empty. They will be removed in a
later patch to reduce churn.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/269171496f1f5f22afa621695bded22976c9d48d.1615552867.git.christophe.leroy@csgroup.eu