hw/arm/armv7m_nvic: Check subpriority in nvic_recompute_state_secure()

hw/arm/armv7m_nvic: Check subpriority in nvic_recompute_state_secure()

Rule R_CQRV says that if two pending interrupts have the same
group priority then ties are broken by looking at the subpriority.
We had a comment describing this but had forgotten to actually
implement the subpriority comparison. Correct the omission.

(The further tie break rules of "lowest exception number" and
"secure before non-secure" are handled implicitly by the order
in which we iterate through the exceptions in the loops.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190430131439.25251-2-peter.maydell@linaro.org