nvic: In escalation to HardFault, support HF not being priority -1

When escalating to HardFault, we must go into Lockup if we
can't take the synchronous HardFault because the current
execution priority is already at or below the priority of
HardFault. In v7M HF is always priority -1 so a simple < 0
comparison sufficed; in v8M the priority of HardFault can
vary depending on whether it is a Secure or NonSecure
HardFault, so we must check against the priority of the
HardFault exception vector we're about to use.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-13-git-send-email-peter.maydell@linaro.org
