core:vsx fix build failure on GCC<=6 due implementation of v_reduce_sum(v_float64x2)

modules/core/include/opencv2/core/hal/intrin_vsx.hpp

@@ -718,7 +718,7 @@ OPENCV_HAL_IMPL_VSX_REDUCE_OP_4(v_float32x4, vec_float4, float, min, vec_min)
 
 inline double v_reduce_sum(const v_float64x2& a)
 {
-    return vec_extract(vec_add(a.val, vec_sld(a.val, a.val, 8)), 0);
+    return vec_extract(vec_add(a.val, vec_permi(a.val, a.val, 3)), 0);
 }
 
 #define OPENCV_HAL_IMPL_VSX_REDUCE_OP_8(_Tpvec, _Tpvec2, scalartype, suffix, func) \