rustdoc: link to stable/beta docs consistently in documentation

 ## User-facing changes

- Intra-doc links to primitives that currently go to rust-lang.org/nightly/std/primitive.x.html will start going to channel that rustdoc was built with. Nightly will continue going to /nightly; Beta will link to /beta; stable compilers will link to /1.52.1 (or whatever version they were built as).
- Cross-crate links from std to core currently go to /nightly unconditionally. They will start going to /1.52.0 on stable channels (but remain the same on nightly channels).
- Intra-crate links from std to std (or core to core) currently go to the same URL they are hosted at; they will continue to do so. Notably, this is different from everything else because it can preserve the distinction between /stable and /1.52.0 by using relative links.

Note that "links" includes both intra-doc links and rustdoc's own
automatically generated hyperlinks.

 ## Implementation changes

- Update the testsuite to allow linking to /beta and /1.52.1 in docs
- Use an html_root_url for the standard library that's dependent on the channel

  This avoids linking to nightly docs on stable.

- Update rustdoc to use channel-dependent links for primitives from an
  unknown crate

- Set DOC_RUST_LANG_ORG_CHANNEL from bootstrap to ensure it's in sync
- Include doc.rust-lang.org in the channel

Author: jyn514

library/alloc/src/lib.rs

@@ -59,7 +59,6 @@
 #![allow(unused_attributes)]
 #![stable(feature = "alloc", since = "1.36.0")]
 #![doc(
-    html_root_url = "https://doc.rust-lang.org/nightly/",
     html_playground_url = "https://play.rust-lang.org/",
     issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/",
     test(no_crate_inject, attr(allow(unused_variables), deny(warnings)))

library/core/src/lib.rs

@@ -51,7 +51,6 @@
 #![cfg(not(test))]
 #![stable(feature = "core", since = "1.6.0")]
 #![doc(
-    html_root_url = "https://doc.rust-lang.org/nightly/",
     html_playground_url = "https://play.rust-lang.org/",
     issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/",
     test(no_crate_inject, attr(deny(warnings))),