tcg: update the cpu running flag in cpu_exec_step_atomic

tcg: update the cpu running flag in cpu_exec_step_atomic

The cpu_exec_step_atomic() function is called with the cpu->running
clear and proceeds to run target code without setting this flag. If
this target code generates an exception then handle_cpu_signal() will
unnecessarily abort.  For example if atomic code generates a memory
protection fault.

This patch at least sets and clears this running flag, and adds some
assertions to help detect other cases.

Signed-off-by: Douglas Crosher <dtc-ubuntu@scieneer.com>
Message-Id: <a272c656-f7c5-019d-1cc0-499b8f80f2fc@scieneer.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>