```console
$ cat >>config.toml <<"END"
profile = "compiler"

[build]
build-stage = 1
host = ["x86_64-unknown-linux-gnu"]
target = ["x86_64-unknown-linux-gnu", "sparc-unknown-none-elf"]
END

$ ./x build  --stage 1
$ rustup toolchain link stage1 build/host/stage1
```

That's probably just building rustc and not cargo.