Add Xtensa toolchain github workflow. (#125)

BUG=http://b/190108540

.github/workflows/xtensa.yml

+on:
+  pull_request_target:
+    types: [labeled]
+    branches:
+      - main
+
+  schedule:
+    # 10am UTC is 3am or 4am PT depending on daylight savings.
+    - cron: '0 10 * * *'
+
+  # Allow manually triggering of the workflow.
+  workflow_dispatch: {}
+
+jobs:
+  unit_tests:
+    runs-on: ubuntu-latest
+
+    if: |
+      github.event_name == 'workflow_dispatch' ||
+      (github.event_name == 'pull_request' && contains(github.event.pull_request.labels.*.name, 'ci:run')) ||
+      (github.event_name == 'schedule' && github.repository == 'tensorflow/tflite-micro')
+
+    name: Fusion F1 Unit Tests (presubmit)
+    steps:
+      - uses: actions/checkout@v2
+      - run: |
+          echo ${{ secrets.TFLM_BOT_PACKAGE_READ_TOKEN }} | docker login ghcr.io -u tflm-bot --password-stdin
+          docker run -it --rm -v `pwd`:/opt/tflite-micro ghcr.io/tflm-bot/xtensa:latest /opt/tflite-micro/tensorflow/lite/micro/tools/ci_build/test_xtensa_fusion_f1.sh

tensorflow/lite/micro/tools/ci_build/test_xtensa_fusion_f1.sh

+#!/usr/bin/env bash
+# Copyright 2021 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ROOT_DIR=${SCRIPT_DIR}/../../../../..
+cd "${ROOT_DIR}"
+pwd
+
+source tensorflow/lite/micro/tools/ci_build/helper_functions.sh
+
+readable_run make -f tensorflow/lite/micro/tools/make/Makefile clean
+readable_run make -f tensorflow/lite/micro/tools/make/Makefile \
+  TARGET=xtensa \
+  TARGET_ARCH=fusion_f1 \
+  OPTIMIZED_KERNEL_DIR=xtensa \
+  XTENSA_CORE=F1_190305_swupgrade \
+  test -j$(nproc)