target/ppc: Use env_archcpu() in helper_book3s_msgsndp()

target/ppc: Use env_archcpu() in helper_book3s_msgsndp()

When CPUArchState* is available (here CPUPPCState*), we
can use the fast env_archcpu() macro to get ArchCPU* (here
PowerPCCPU*). The QOM cast POWERPC_CPU() macro will be
slower when building with --enable-qom-cast-debug.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20231009110239.66778-2-philmd@linaro.org>