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>

target/ppc/excp_helper.c

@@ -3136,7 +3136,7 @@ void helper_book3s_msgclrp(CPUPPCState *env, target_ulong rb)
 void helper_book3s_msgsndp(CPUPPCState *env, target_ulong rb)
 {
     CPUState *cs = env_cpu(env);
-    PowerPCCPU *cpu = POWERPC_CPU(cs);
+    PowerPCCPU *cpu = env_archcpu(env);
     CPUState *ccs;
     uint32_t nr_threads = cs->nr_threads;
     int ttir = rb & PPC_BITMASK(57, 63);