arm: Factor out "generate right kind of step exception"

arm: Factor out "generate right kind of step exception"

We currently have two places that do:
            if (dc->ss_active) {
                gen_step_complete_exception(dc);
            } else {
                gen_exception_internal(EXCP_DEBUG);
            }

Factor this out into its own function, as we're about to add
a third place that needs the same logic.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1491844419-12485-4-git-send-email-peter.maydell@linaro.org