target-arm: Fix unreachable code in gicv3_class_name()

Coverity complains that the exit() in gicv3_class_name()
can be unreachable, because if TARGET_AARCH64 is defined
then all code paths return before reaching it. Move the
exit() up to the error_report() that it belongs with.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1468260552-8400-1-git-send-email-peter.maydell@linaro.org