Add reference fallback for TARGET_ARCH!=hifimini
Add reference fallback for TARGET_ARCH!=hifimini

A follow-up change will add in calls to the XaNNLib to get an optimized
softmax implementation.

Tested that keyword_benchmark has a latency increase of ~1000 ticks for
the Fusion F1. This latency improvement does not matter since the
current change is setting the stage for pulling in the optimized
implementation.

This command:
```
make -f tensorflow/lite/micro/tools/make/Makefile TARGET=xtensa OPTIMIZED_KERNEL_DIR=xtensa TARGET_ARCH=fusion_f1 XTENSA_CORE=F1_190305_swupgrade test_keyword_benchmark -j8
```

before this change:
```
InitializeKeywordRunner() took 279548 ticks (279 ms)
KeywordRunNIerations(1) took 151249 ticks (151 ms)
KeywordRunNIerations(10) took 1511997 ticks (1511 ms)
```

after this change:
```
InitializeKeywordRunner() took 201464 ticks (201 ms)
KeywordRunNIerations(1) took 152158 ticks (152 ms)
KeywordRunNIerations(10) took 1521087 ticks (1521 ms)
```

Progress towards http://b/177457688

yongtang/tensorflow,gautam1858/tensorflow,Intel-Corporation/tensorflow,gautam1858/tensorflow,karllessard/tensorflow,annarev/tensorflow,sarvex/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow-pywrap_tf_optimizer,paolodedios/tensorflow,Intel-tensorflow/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-experimental_link_static_libraries_once,Intel-tensorflow/tensorflow,sarvex/tensorflow,petewarden/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_saved_model,karllessard/tensorflow,sarvex/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow,annarev/tensorflow,karllessard/tensorflow,karllessard/tensorflow,frreiss/tensorflow-fred,frreiss/tensorflow-fred,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,petewarden/tensorflow,petewarden/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,karllessard/tensorflow,petewarden/tensorflow,frreiss/tensorflow-fred,annarev/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,Intel-Corporation/tensorflow,petewarden/tensorflow,Intel-Corporation/tensorflow,Intel-Corporation/tensorflow,paolodedios/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,sarvex/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow,karllessard/tensorflow,Intel-Corporation/tensorflow,petewarden/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,yongtang/tensorflow,gautam1858/tensorflow,Intel-Corporation/tensorflow,sarvex/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,tensorflow/tensorflow,frreiss/tensorflow-fred,petewarden/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_saved_model,karllessard/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow-experimental_link_static_libraries_once,paolodedios/tensorflow,yongtang/tensorflow,annarev/tensorflow,annarev/tensorflow,Intel-tensorflow/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow,Intel-Corporation/tensorflow,frreiss/tensorflow-fred,yongtang/tensorflow,gautam1858/tensorflow,paolodedios/tensorflow,yongtang/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_tf_optimizer,sarvex/tensorflow,sarvex/tensorflow,tensorflow/tensorflow-pywrap_saved_model,annarev/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,Intel-tensorflow/tensorflow,karllessard/tensorflow,gautam1858/tensorflow,Intel-tensorflow/tensorflow,annarev/tensorflow,annarev/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,paolodedios/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_saved_model,yongtang/tensorflow,Intel-Corporation/tensorflow,petewarden/tensorflow,paolodedios/tensorflow,gautam1858/tensorflow,frreiss/tensorflow-fred,annarev/tensorflow,petewarden/tensorflow,paolodedios/tensorflow,gautam1858/tensorflow,yongtang/tensorflow,paolodedios/tensorflow,sarvex/tensorflow,paolodedios/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,petewarden/tensorflow,paolodedios/tensorflow,annarev/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,yongtang/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,paolodedios/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,Intel-tensorflow/tensorflow,tensorflow/tensorflow,annarev/tensorflow,yongtang/tensorflow,Intel-tensorflow/tensorflow,yongtang/tensorflow,Intel-tensorflow/tensorflow,yongtang/tensorflow,karllessard/tensorflow