This is a temporary fix to get rustc building again with these
configurations (which notably include the default aarch64-darwin one)
without causing a stdenv rebuild. The real fix will be to avoid
passing these options through the Clang wrapper when the target is
overridden.
It could be that there are packages that need wasm32-unknown-unknown
beyond those I've marked as broken here — it's impossible to be sure
without a full rebuild. But this should be most of them.
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.
