C++11 leak memory on `emscripten_exit_with_live_runtime`

In fact any C++ objects on the stack when `emscripten_exit_with_live_runtime` will leak unless explictly freed.

C++11 thread functions have a unique pointer in their main entry point:

https://github.com/emscripten-core/emscripten/blob/719b7c6a044ea1193cd602a17bb437f1b21dc931/system/lib/libcxx/include/thread#L290

Calling `emscripten_exit_with_live_runtime` from within a thread is unusual, and the load is just 8 bytes so its probably not urgent to fix.