wasm cross-compiler doesn't compile with `+werror`

It appears that the Wasm backend isn't -Werror clean. Specifically, this fails:

$ nix develop ghc.nix#devShells.x86_64-linux.wasi-cross
$ ./boot
$ configure_ghc
$ hadrian/build --flavour=default+werror

The first errors that are encountered are a number of RTS warnings.

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