New build flag: USE_CPU_EXTENSIONS (#328)

**Issue:**
Some customers are encountering mysterious crashes in some of these functions. Give them the ability to disable them while we attempt to diagnose the issue.

**Description of changes:**
Set `-DUSE_CPU_EXTENSIONS=OFF` to disable CPU-specific functions. This build flag is `ON` by default.

This feature comes by updating submodule aws-crt-cpp: v0.17.0 -> v0.17.1

.github/workflows/ci.yml

@@ -66,6 +66,15 @@ jobs:
         aws s3 cp s3://aws-crt-test-stuff/ci/${{ env.BUILDER_VERSION }}/linux-container-ci.sh ./linux-container-ci.sh && chmod a+x ./linux-container-ci.sh
         ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --cmake-extra=-DBYO_CRYPTO=ON skip_samples=1
 
+  linux-no-cpu-extensions:
+    runs-on: ubuntu-latest
+    steps:
+        # We can't use the `uses: docker://image` version yet, GitHub lacks authentication for actions -> packages
+    - name: Build ${{ env.PACKAGE_NAME }}
+      run: |
+        aws s3 cp s3://aws-crt-test-stuff/ci/${{ env.BUILDER_VERSION }}/linux-container-ci.sh ./linux-container-ci.sh && chmod a+x ./linux-container-ci.sh
+        ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --cmake-extra=-DUSE_CPU_EXTENSIONS=OFF
+
   windows-vs16:
     runs-on: windows-latest
     steps:
@@ -95,6 +104,16 @@ jobs:
           python -c "from urllib.request import urlretrieve; urlretrieve('${{ env.BUILDER_HOST }}/${{ env.BUILDER_SOURCE }}/${{ env.BUILDER_VERSION }}/builder.pyz?run=${{ env.RUN }}', 'builder.pyz')"
           python builder.pyz build -p ${{ env.PACKAGE_NAME }} --spec=downstream
 
+  windows-no-cpu-extensions:
+    runs-on: windows-latest
+    steps:
+      - name: Build ${{ env.PACKAGE_NAME }} + consumers
+        run: |
+          md D:\a\work
+          cd D:\a\work
+          python -c "from urllib.request import urlretrieve; urlretrieve('${{ env.BUILDER_HOST }}/${{ env.BUILDER_SOURCE }}/${{ env.BUILDER_VERSION }}/builder.pyz', 'builder.pyz')"
+          python builder.pyz build -p ${{ env.PACKAGE_NAME }} --cmake-extra=-DUSE_CPU_EXTENSIONS=OFF
+
   osx:
     runs-on: macos-latest
     steps: