Move target-agnostic declarations from "internal-target.h"
to a new "internal-common.h" header.
monitor.c now don't include target specific headers and can
be compiled once in system_ss[].

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20230914185718.76241-10-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>