target/arm: Add missing entries to excnames[] for log strings

Recent changes have added new EXCP_ values to ARM but forgot
to update the excnames[] array which is used to provide
human-readable strings when printing information about the
exception for debug logging. Add the missing entries, and
add a comment to the list of #defines to help avoid the mistake
being repeated in future.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1491486340-25988-1-git-send-email-peter.maydell@linaro.org