simplify probabilistic rounding default impl, similar to the NEON impl
simplify probabilistic rounding default impl, similar to the NEON impl

maciekcc/gemmlowp,dave-andersen/gemmlowp,google/gemmlowp,petewarden/gemmlowp,maciekcc/gemmlowp,maciekcc/gemmlowp,google/gemmlowp,google/gemmlowp,efeguney/gemmlowp,miaowang14/gemmlowp,petewarden/gemmlowp,efeguney/gemmlowp,miaowang14/gemmlowp,miaowang14/gemmlowp,google/gemmlowp,dave-andersen/gemmlowp,petewarden/gemmlowp,efeguney/gemmlowp,petewarden/gemmlowp,dave-andersen/gemmlowp