powerpc/32: Add EXCEPTION_PROLOG_0 in head_32.h

This patch creates a macro for the very first part of
exception prolog, this will help when implementing
CONFIG_VMAP_STACK

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/2249fe62f481121a180e9655ad2b998093f318f3.1576916812.git.christophe.leroy@c-s.fr