added v_reduce_sum4() universal intrinsic; corrected number of threads in cv::getNumThreads() in the case of GCD

There is no cast to wide integer type:
    std::numeric_limits<ST>::max() * std::numeric_limits<ST>::max()

Fixed snprintf for VS 2013 (#8816)

* Fixed snprintf for VS 2013

* snprintf: removed declaration from header, changed implementation

* cv_snprintf corrected according to comments

* update snprintf patch

C4189: 'clImageUV' : local variable is initialized but not referenced

 * avoid link error (move the implementation of software version to header)
 * make getConvertFuncFp16 local (move from precomp.hpp to convert.hpp)
 * fix error on 32bit x86