Allow emscripten_fetch in Asyncify

Currently there is a perceived inconsistency between APIs like `emscripten_sleep` and `emscripten_fetch`: while both perform sort of an async I/O, one of them works with Asyncify, while another works only by compiling code in a separate thread.

Technically, there is no reason for this discrepancy, and it should be possible to allow APIs like `emscripten_fetch` with just Asyncify if user wants it to run on the main thread instead of a separate one.

Perhaps this is a more general issue though and there should be some shared mechanism for all such APIs to choose between Asyncify and thread-based blocking depending on compile flags?