powerpc/syscall: Make interrupt.c buildable on PPC32

To allow building interrupt.c on PPC32, ifdef out specific PPC64
code or use helpers which are available on both PP32 and PPC64

Modify Makefile to always build interrupt.o

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ba073ad67bd971a88ce331b65d6655523b54c794.1612796617.git.christophe.leroy@csgroup.eu