target-arm: Fix gdb singlestep handling in arm_debug_excp_handler()

Do not raise a CPU exception if no CPU breakpoint has fired, since
singlestep is also done by generating a debug internal exception. This
fixes a bug with singlestepping in gdbstub.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1446726361-18328-1-git-send-email-serge.fdrv@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>