target/ppc: powerpc_excp: Group unimplemented exceptions

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220107222601.4101511-4-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>