boostrap: skip no_std targets in Std doc step This fixes a bug that currently prevents us from adding no_std library targets to rustc in nixpkgs (https://github.com/NixOS/nixpkgs/pull/382166). When running `./x.py doc`, the `Std` doc step generally fails for no_std targets, logs: https://gist.github.com/niklaskorz/fb83f9503ce19b75e8b1af02cdebd592 Skipping no_std targets in this step will allow using no_std targets such as `bpfel-unknown-none` together with other targets in the same config without blocking the doc generator for them, e.g. ``` ./configure --release-channel=stable --tools=rustc,rustdoc,rust-analyzer-proc-macro-srv --build=aarch64-apple-darwin --host=aarch64-apple-darwin --target=aarch64-apple-darwin,bpfel-unknown-none ./x.py doc ``` Logs with this fix applied: https://gist.github.com/niklaskorz/cdd50aaea33ede579f737434286d800b