https://forge.rust-lang.org/infra/other-installation-methods.html#standalone, https://static.rust-lang.org/dist/rust-std-1.47.0-armv7-unknown-linux-gnueabi.tar.xz. It looks like this is generated by https://github.com/rust-lang/rust-forge/blob/a91984837ddc87571e9dde6c90bd274c1c750f13/blacksmith/src/lib.rs#L140, but I can't see where `rustup` is populated.