target/s390x: Use env_archcpu() in handle_diag_308()

target/s390x: Use env_archcpu() in handle_diag_308()

When CPUArchState* is available (here CPUS390XState*), we
can use the fast env_archcpu() macro to get ArchCPU* (here
S390CPU*). The QOM cast S390_CPU() macro will be slower when
building with --enable-qom-cast-debug.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20231009110239.66778-4-philmd@linaro.org>