target/riscv: Use env_archcpu() in [check_]nanbox()

target/riscv: Use env_archcpu() in [check_]nanbox()

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

Inspired-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20231009110239.66778-3-philmd@linaro.org>