include/exec: Split out exec/cpu-interrupt.h

Some of these bits are actually common to all cpus; while the
reset have common reservations for target-specific usage.
While generic code cannot know what the target-specific usage is,
common code can know what to do with the bits, e.g. single-step.

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