target-arm: Fix 32 bit signed saturating narrow

The returned value when doing saturating signed 64->32 bit
conversion of a negative number was incorrect due to a missing cast.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-arm/neon_helper.c

@@ -1209,7 +1209,7 @@ uint32_t HELPER(neon_narrow_sat_s32)(CPUState *env, uint64_t x)
 {
     if ((int64_t)x != (int32_t)x) {
         SET_QC();
-        return (x >> 63) ^ 0x7fffffff;
+        return ((int64_t)x >> 63) ^ 0x7fffffff;
     }
     return x;
 }