Pass semihosting exit code back to system.

In order to run unit tests under semihosting, it is necessary to pass the
application exit code back to the system.

ARM defines only the code to be used for non-error application exit
(ADP_Stopped_ApplicationExit), all other codes should return non-zero
exit codes.

This patch checks if the application code passed via TARGET_SYS_EXIT is
ADP_Stopped_ApplicationExit, and return 0, otherwise return 1.

Signed-off-by: Liviu Ionescu <ilg@livius.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
