target-arm: Report S/NS status in the CPU debug logs

If this CPU supports EL3, enhance the printing of the current
CPU mode in debug logging to distinguish S from NS modes as
appropriate.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1445883178-576-3-git-send-email-peter.maydell@linaro.org