target/arm: v8M: Check state of exception being returned from

In v8M, an attempt to return from an exception which is not
active is an illegal exception return. For this purpose,
exceptions which can configurably target either Secure or
NonSecure are not considered to be active if they are
configured for the opposite security state for the one
we're trying to return from (eg attempt to return from
an NS NMI but NMI targets Secure). In the pseudocode this
is handled by IsActiveForState().

Detect this case rather than counting an active exception
possibly of the wrong security state as being sufficient.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190617175317.27557-4-peter.maydell@linaro.org