Broken docs.rs builds on Windows

[kvark]

See the logs in https://docs.rs/crate/gfx-backend-dx12/0.5.9/builds/281262
See also gfx-rs/gfx#2957

[grovesNL]

Cross-compilation from Linux will be necessary for Windows and macOS targets due to rust-lang/docs.rs#1015

We can probably make spirv_cross work with cross-compilation, but we'd need to figure out how to configure cc for docs.rs. Other crates depending on cc have similar issues (e.g. https://docs.rs/crate/objc_exception/0.1.2/builds/200687) so we'd have to look into what other crates are doing.