Buildbot emscripten cores do not have async support

[JoeOsborn]

It looks like none of the emscripten cores in https://buildbot.libretro.com/nightly/emscripten/ are compiled with the correct emscripten flags. A quick text search through e.g. fceumm_libretro.js for emscripten_sleep shows that it just throws an exception ("Please compile your program with async support") rather than sleeping. Async support seems to be required since #15479. My guess is that the core makefiles aren't passing along the right flags?

The web player at web.libretro.com also seems busted, in that retroarch won't seem to boot into the menu for the fceumm core in Linux Firefox or Chromium. I don't know if this is a related issue.

[JoeOsborn]

I think the fix here is to default ASYNC to 1 instead of 0 in Retroarch’s Makefile.emscripten, possibly only if HAVE_OPENAL is 1.