Since wasm32-unknown-unknown doesn't require any extra
platform-specific dependencies (e.g. libc), we might as well just
always include in with rustc. We could also include other targets
fitting these criteria, but I wasn't able to find any others that
actually build (not even wasm64-unknown-unknown).
The old rustc-wasm32 package disabled documentation, but we don't
actually need to do that — we just need to allow for some broken links
in the wasm32-unknown-unknown documentation. Broken links in
documentation are an upstream issue anyway. There's no need we need
to fail our build for them.
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