Auto merge of #136586 - Kobzol:lto-rustdoc-fix-stage-1, r=onur-ozkan

Only apply LTO to rustdoc at stage 2

It doesn't make much sense at stage 1, and it was broken anyway. This was implemented in rust-lang/rust#135832. The issue with LTO and stage 1 rustdoc was reported [here](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/x.20test.20with.20lto.20.3D.20.22thin.22.20fails.20to.20build.20rustdoc.3F).

r? `@onur-ozkan`

Author: bors