Default to single-threading. Explanation in comment.
Default to single-threading. Explanation in comment.

petewarden/gemmlowp,petewarden/gemmlowp,maciekcc/gemmlowp,google/gemmlowp,google/gemmlowp,google/gemmlowp,maciekcc/gemmlowp,maciekcc/gemmlowp,google/gemmlowp,petewarden/gemmlowp,petewarden/gemmlowp