Move static allocation for fetch queue from JS to C++ code

[sbc100]

This moves the function emscripten_get_fetch_queue and its static
allocation in the native code avoiding the runtime allocation on the JS side.

See: #12040

[sbc100]

Should we wait for @juj to weigh in? This seems like a pretty straight forward transformation, but I could be missing something subtle.

@kripken feel free to land if you need to change for your work.

[kripken]

It would be good to get @juj's feedback, but I think this is good to land whenever you want. Seems simple and correct to me, passes tests, and in the worst case we can revert it later (easily I think, since there is no stack of changes expected to land on top of these files AFAIK).

(I don't need this urgently myself, but it's a great change.)