build: add llvm-tools to manifest
This commit expands on a previous commit to build llvm-tools as a rustup component. It causes the llvm-tools component to be built if the extended step is active. It also adds llvm-tools to the build manifest so rustup can find it.
I tested this as far as I could, but had to hack `build-manifest/src/main.rs` a bit as it is not supported on MacOS. The main change I am not sure about is this line:
```rust
self.package("llvm-tools", &mut manifest.pkg, TARGETS);
```
There are numerous calls to `self.package()`, and I'm not sure if `TARGETS`, `HOSTS`, or `["*"]` is appropriate for llvm-tools.
Otherwise I mostly copied the example set by `rustfmt-preview`.
b7c6e8f180