target/rx: Use cpu_psw_z as temp in flags computation

target/rx: Use cpu_psw_z as temp in flags computation

Since PSW_Z = PSW_S, we can move that assignment to the end
and use PSW_Z as a temporary while computing PSW_O.
Use tcg_constant_i32 instead of tcg_const_i32.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>