nvic: Add cached vectpending_prio state

Instead of looking up the pending priority
in nvic_pending_prio(), cache it in a new state struct
field. The calculation of the pending priority given
the interrupt number is more complicated in v8M with
the security extension, so the caching will be worthwhile.

This changes nvic_pending_prio() from returning a full
(group + subpriority) priority value to returning a group
priority. This doesn't require changes to its callsites
because we use it only in comparisons of the form
  execution_prio > nvic_pending_prio()
and execution priority is always a group priority, so
a test (exec prio > full prio) is true if and only if
(execprio > group_prio).

(Architecturally the expected comparison is with the
group priority for this sort of "would we preempt" test;
we were only doing a test with a full priority as an
optimisation to avoid the mask, which is possible
precisely because the two comparisons always give the
same answer.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-5-git-send-email-peter.maydell@linaro.org
