Use protected visibility when building rustc with LLD https://github.com/rust-lang/compiler-team/issues/782 I wasn't sure about having two commits in a PR, but I figured, at least initially it might make sense to discuss these commits together. Happy to squash, or move the second commit to a separate PR. I contemplated trying to enable protected visibility for more cases when LLD will be used other than just `-Zlinker-features=+lld`, but that would be more a complex change that probably still wouldn't cover all cases when LLD is used, so went with the simplest option of just checking if the linker-feature is enabled. r? lqd