bootstrap: support x.py build src/librustdoc

[jyn514]

Current behavior:

thread 'main' panicked at 'Error: no rules matched src/librustdoc.', src/bootstrap/builder.rs:222:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failed to run: /home/joshua/src/rust/build/bootstrap/debug/bootstrap build src/librustdoc

Expected behavior: This builds rustdoc.

I'm interested in working on this, if anyone has suggestions that would be much appreciated. I found src/bootstrap/builder.rs but I'm having a hard time understanding ShouldRun.

[jyn514]

It looks like the existing (undocumented) way to do this is x.py build src/tools/rustdoc. I'm going to try and add src/librustdoc as an alias for that (since no one actually works with src/tools/librustdoc, it's almost empty).

[jyn514]

Ok, undocumented was a lie, it's here, just hard to find: https://rustc-dev-guide.rust-lang.org/rustdoc.html#cheat-sheet