accel/tcg: Make icount.o a target agnostic unit

Remove the unused "exec/exec-all.h" header. There is
no more target specific code in it: make it target
agnostic (rename using the '-common' suffix). Since
it is TCG specific, move it to accel/tcg, updating
MAINTAINERS.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20230914185718.76241-11-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>