target/ppc: Use ppc_interrupts_little_endian in powerpc_excp

The ppc_interrupts_little_endian function is now suitable for
determining the endianness of interrupts for all CPUs.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220107222601.4101511-7-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>