Use `PYODIDE` environment variable for Emscripten cross-compilation detection (#21714)

I noticed that y'all were carrying this workaround ever since
#14888. A better way to detect this is through [the `PYODIDE`
environment variable, which we've documented over the
years](https://pyodide-build.readthedocs.io/en/latest/how-to/migrate.html#detecting-pyodide-at-build-time),
and is what most projects use now. The MACHDEP generality would mostly
apply to non-Pyodide-specific WASM build targets, which are growing in
relevance but are a bit less mature as Pyodide at this time.