Building wasm32-unknown-emscripten fails since it includes both wasm and linux platform

[Wumpf]

Worked out of the box in 0.27.5, but in 0.28.0 it seems that the config checks in https://github.com/rust-windowing/winit/blob/master/src/platform_impl/mod.rs#L9 incorrectly detect wasm32-unknown-emscripten as a linux platform, causing compilation failure

See https://github.com/gfx-rs/wgpu/actions/runs/5618255769/job/15223561268?pr=3950#step:9:55

[daxpedda]

I wasn't aware that wasm32-unknown-emscripten enables cfg(unix), that's funny.
In any case, we don't support Emscripten.

The reason why this used to work, is because wasm-bindgen stubs out implementations on any unsupported platforms, it's just that any calls would panic.

@kchibisov I would argue that Winit, because it doesn't support Emscripten, should just fail on compilation. As is already done with other unsupported platforms here:
https://github.com/rust-windowing/winit/blob/43acf7f42f4474d2c66815e9c1b392b4d3175594/src/platform_impl/mod.rs#L57-L68
WDYT?

[kchibisov]

@kchibisov I would argue that Winit, because it doesn't support Emscripten, should just fail on compilation. As is already done with other unsupported platforms here:

Sounds good to me.