fix time source for wasm

[DenizUgur]

In wasm build, usage of gettimeofday() is painfully slow when gpac runs on a worker. Emscripten offers a substitute for it. I had to make this change because when I profiled my application, almost all of the time wasm made a call to javascript to get the time and it stalled the entire stack because of it.

I haven't fully tested it but as emscripten documentation pointed out, this might use performance.now(). Which means between different workers (threads) this value will have a different origin (mdn reference). Depending on how these clocks are used internally the implementation might need to change. For reference, here is how this function is defined in emscripten:

https://github.com/emscripten-core/emscripten/blob/main/system/lib/standalone/standalone.c#L179-L185

fixes #2775
related urho3d/urho3d#916

[jeanlf]

According to the emscripten worker doc, using emscripten_get_now() should be time-aligned in pthreads, so I think we can merge. Thanks !