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.
