target-arm: Make arm_current_el() return sensible values for M profile

Although M profile doesn't have the same concept of exception level
as A profile, it does have a notion of privileged versus not, which
we currently track in the privmode TB flag. Support returning this
information if arm_current_el() is called on an M profile core, so
that we can identify the correct MMU index to use (and put the MMU
index in the TB flags) without having to special-case M profile.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Greg Bellows <greg.bellows@linaro.org>

target-arm/cpu.h

@@ -1218,6 +1218,10 @@ static inline bool cptype_valid(int cptype)
  */
 static inline int arm_current_el(CPUARMState *env)
 {
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        return !((env->v7m.exception == 0) && (env->v7m.control & 1));
+    }
+
     if (is_a64(env)) {
         return extract32(env->pstate, 2, 2);
     }