target/arm: Add state field, feature bit and migration for v8M secure state

As the first step in implementing ARM v8M's security extension:
 * add a new feature bit ARM_FEATURE_M_SECURITY
 * add the CPU state field that indicates whether the CPU is
   currently in the secure state
 * add a migration subsection for this new state
   (we will add the Secure copies of banked register state
   to this subsection in later patches)
 * add a #define for the one new-in-v8M exception type
 * make the CPU debug log print S/NS status

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1503414539-28762-4-git-send-email-peter.maydell@linaro.org
