ci: test x64 machine codegen

.github/workflows/ci.yml

@@ -44,7 +44,9 @@ jobs:
 #    - name: Test v->js
 #      run: ./v -o hi.js examples/hello_v_js.v && node hi.js
     - name: Bare metal
-      run: ./v -bare -o bare vlib/os/bare/bare_example_linux.v
+      run: ./v -freestanding -o bare vlib/os/bare/bare_example_linux.v
+    - name: x64 machine code generation
+      run: ./v -x64 examples/x64/hello_world.v
 
   build-ubuntu-tcc:
     runs-on: ubuntu-18.04