* Add Hexagon optimized kernels.

 * Hexagon optimized kernels copied from https://source.codeaurora.org/quic/embedded_ai/tensorflow at 2d052806c211144875c89315a4fc6f1393064cf6
 * Changed the include paths and directory structure a bit.
 * Modified Makefile to allow optimized kernels to be in a separate directory
 * Path to the Hexagon lib is now specified on the command line.

Verified that the optimized kernels are properly built and linked with:

```
make -f tensorflow/lite/micro/tools/make/Makefile TARGET=hexagon OPTIMIZED_KERNEL_DIR=hexagon OPTIMIZED_KERNEL_DIR_PREFIX=third_party HEXAGON_TFLM_LIB=~/Qualcomm/tflm_google/hexagon_tflm_core.a  -j8 run_keyword_benchmark
```

Gives:
```
KeywordRunNIerations(1) took 52608 ticks (52 ms)
```

Whereas reference kernels with:
```
make -f tensorflow/lite/micro/tools/make/Makefile TARGET=hexagon  -j8 run_keyword_benchmark
```

Gives:
```
KeywordRunNIerations(1) took 110248 ticks (110 ms)
```

BUG=http://b/190754463

* enable code style checks for third_party as well.

* clang-formatted the hexagon kernels.

* Rename to keep scope focused on what we currently need.