target/ppc: Introduce a wrapper for powerpc_excp

Next patches will split powerpc_excp in multiple family specific
handlers. This patch adds a wrapper to make the transition clearer.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220107222601.4101511-8-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>