target/ppc: powerpc_excp: Stop passing excp_model around

We can just access it directly in powerpc_excp.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
[ clg: Took into account removal of inline ]
Message-Id: <20211229165751.3774248-6-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>