CI: generate ABI files if changed

So commit author can just download them as
artifacts and commit.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #12379

.github/workflows/checkstyle.yaml

@@ -31,6 +31,20 @@ jobs:
       run: |
         make lint
     - name: CheckABI
+      id: CheckABI
       run: |
         make -j$(nproc)
         make checkabi
+    - name: StoreABI
+      if: failure() && steps.CheckABI.outcome == 'failure'
+      run: |
+        make storeabi
+    - name: Prepare artifacts
+      if: failure() && steps.CheckABI.outcome == 'failure'
+      run: |
+        find -name *.abi | tar -cf abi_files.tar -T -
+    - uses: actions/upload-artifact@v2
+      if: failure() && steps.CheckABI.outcome == 'failure'
+      with:
+        name: New ABI files (use only if you're sure about interface changes)
+        path: abi_files.tar