Add emcc build (stuck if all cpus used on GH)

.github/workflows/emcc.yml

@@ -0,0 +1,20 @@
+name: Emscripten Build
+
+on: [push, pull_request]
+
+jobs:
+  emcc:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: mymindstorm/setup-emsdk@v11
+      - uses: actions/checkout@v2
+      - name: Cache sources
+        id: cache-sources
+        uses: actions/cache@v2
+        with:
+          path: .
+          key: cache-yosys
+      - name: Build
+        run: |
+          make config-emcc
+          make