Implement openmp threads

[liyinhgqw]

Issue: #148

[bjacob]

Thanks for the interesting pull request. We are not currently using OpenMP, and so our interest in taking in a change that makes source code even moderately more complex to improve support with OpenMP, is low. I also do not have much time at the moment to get deep enough into this to really make a good review for this PR. For now, my best course of action is to close this PR, since I don't have a short-term plan to merge it, but its code will be useful to keep around for reference, should the topic of OpenMP support arise again. Meanwhile, if you keep a GitHub fork of gemmlowp with this code patched in, users who care about OpenMP support will be able to use it, and we can then reconsider if such a fork becomes popular.