[TFLite] Fix gemmlowp CMake integration
GIT_TAG in gemmlowp.cmake wasn't is sync with
tensorflow/third_party/gemmlowp/workspace.bzl, an older version was used.

The new version of gemmlowp has a CMake file in contrib, so use that instead.

gemmlowp's CMake file defines gemmlowp::gemmlowp for the header only library.

All of this makes it possible to switch between gemmlowp being supplied by the system or by tensorflow.