link error: undefined symbol of the EM_JS in EMSDK 3.1.28

I have an EM_JS function defined in individual file and it works just fine before. But recently I upgraded EMSDK to 3.1.28, I received linking error: undefined symbol of the EM_JS function.

I have opened EMCC_DEBUG=1, and got following errors.

```
profiler:DEBUG: block "JS symbol generation" took 0.391 seconds

wasm-ld: error: CMakeFiles/PRJ_dir/Unity/unity_5_cxx.cxx.o: undefined symbol: THE_EM_JS
```

I checked the `__em_js_xxx` symbols existing in another `unity_12_cxx.cxx.o` file. Why the Binaryen failed to handle it?
How can I debug the Binaryen processing or maybe logs?

cc @sbc100 , any ideas?