nvic: Compare group priority for escalation to HF

In armv7m_nvic_set_pending() we have to compare the
priority of an exception against the execution priority
to decide whether it needs to be escalated to HardFault.
In the specification this is a comparison against the
exception's group priority; for v7M we implemented it
as a comparison against the raw exception priority
because the two comparisons will always give the
same answer. For v8M the existence of AIRCR.PRIS and
the possibility of different PRIGROUP values for secure
and nonsecure exceptions means we need to explicitly
calculate the vector's group priority for this check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-12-git-send-email-peter.maydell@linaro.org
