`emcc.py` is not marked as executable in emcripten's git repo so the
build failed when trying to locate emscripten. However, it turns out
that `emcc` is marked as executable, so we use that instead.
On Windows however, we still need to use `emcc.py` because otherwise
Meson won't detect it as valid compiler.
Fixes#8010.
Note that #8398 moves other settings into a shared resources folder, and
once that hits master, it would be good to do another round of
consolidation to share the wasm defs in resources as well.
