Set `--cfg bootstrap` for `rustdoc` when building docs with bootstrap compiler

[ecstatic-morse]

Currently, ./x.py doc --stage 0 src/libstd fails after running into some #[cfg(not(bootstrap))] stuff.

I had to do this manually via RUSTDOCFLAGS, but I think this should be the default, no?

[Mark-Simulacrum]

Yeah, this seems reasonable. I suspect this is just another thing where "stage 0" for most things isn't really natively supported due to lack of testing (in CI, locally, etc.).