target/arm: Initialize exc_secure correctly in do_v7m_exception_exit()

In do_v7m_exception_exit(), we use the exc_secure variable to track
whether the exception we're returning from is secure or non-secure.
Unfortunately the statement initializing this was accidentally
inside an "if (env->v7m.exception != ARMV7M_EXCP_NMI)" conditional,
which meant that we were using the wrong value for NMI handlers.
Move the initialization out to the right place.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180720145647.8810-3-peter.maydell@linaro.org

target/arm/helper.c

@@ -7052,6 +7052,7 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
             /* For all other purposes, treat ES as 0 (R_HXSR) */
             excret &= ~R_V7M_EXCRET_ES_MASK;
         }
+        exc_secure = excret & R_V7M_EXCRET_ES_MASK;
     }
 
     if (env->v7m.exception != ARMV7M_EXCP_NMI) {
@@ -7062,7 +7063,6 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
          * which security state's faultmask to clear. (v8M ARM ARM R_KBNF.)
          */
         if (arm_feature(env, ARM_FEATURE_M_SECURITY)) {
-            exc_secure = excret & R_V7M_EXCRET_ES_MASK;
             if (armv7m_nvic_raw_execution_priority(env->nvic) >= 0) {
                 env->v7m.faultmask[exc_secure] = 0;
             }