[wasm] Qt-5.14 multithreading has broken with emscripten fastcomp backend

    I built qt-5.14 Webassembly multithreading with the following command:

./configure -opensource -confirm-license -xplatform wasm-emscripten -feature-thread -nomake examples -nomake tests -skip qt3d -skip qtdeclarative -skip qtmultimedia -skip qttools -skip qtxmlpatterns

    The emscripten SDK I used is 1.39.4-fastcomp. I found that QThread feature has broken. After some investigation I found that starting from qt-5.14, qthread_unix.cpp will use thread_local keyword to store current thread data, but with emscripten fastcomp backend thread_local keyword doesn't work as expected. This issue will make qt-5.14 wasm multithreading version totally unusable.
For more detailed information of thread_local keyword issue please refer to:
https://github.com/emscripten-core/emscripten/issues/10134

    For now, the only workaround of this issue is to use emscripten's upstream backend, but that will lead to another issue: https://github.com/emscripten-core/emscripten/issues/10131