target/arm: Don't set EXC_RETURN.ES if Security Extension not present

In v7m_exception_taken(), for v8M we set the EXC_RETURN.ES bit if
either the exception targets Secure or if the CPU doesn't implement
the Security Extension.  This is incorrect: the v8M Arm ARM specifies
that the ES bit should be RES0 if the Security Extension is not
implemented, and the pseudocode agrees.

Remove the incorrect condition, so that we leave the ES bit 0
if the Security Extension isn't implemented.

This doesn't have any guest-visible effects for our current set of
emulated CPUs, because all our v8M CPUs implement the Security
Extension; but it's worth fixing in case we add a v8M CPU without
the extension in future.

Reported-by: Igor Kotrasinski <i.kotrasinsk@samsung.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

target/arm/m_helper.c

@@ -879,7 +879,7 @@ static void v7m_exception_taken(ARMCPU *cpu, uint32_t lr, bool dotailchain,
         }
 
         lr &= ~R_V7M_EXCRET_ES_MASK;
-        if (targets_secure || !arm_feature(env, ARM_FEATURE_M_SECURITY)) {
+        if (targets_secure) {
             lr |= R_V7M_EXCRET_ES_MASK;
         }
         lr &= ~R_V7M_EXCRET_SPSEL_MASK;