nvic: Add cached vectpending_is_s_banked state

nvic: Add cached vectpending_is_s_banked state

With banked exceptions, just the exception number in
s->vectpending is no longer sufficient to uniquely identify
the pending exception. Add a vectpending_is_s_banked bool
which is true if the exception is using the sec_vectors[]
array.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1505240046-11454-4-git-send-email-peter.maydell@linaro.org