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

src/test/rustdoc/intra-doc/prim-methods-external-core.rs

@@ -9,8 +9,8 @@
 #![crate_type = "rlib"]
 
 // @has prim_methods_external_core/index.html
-// @has - '//*[@id="main"]//a[@href="https://doc.rust-lang.org/nightly/std/primitive.char.html"]' 'char'
-// @has - '//*[@id="main"]//a[@href="https://doc.rust-lang.org/nightly/std/primitive.char.html#method.len_utf8"]' 'char::len_utf8'
+// @has - '//*[@id="main"]//a[@href="{{channel}}/std/primitive.char.html"]' 'char'
+// @has - '//*[@id="main"]//a[@href="{{channel}}/std/primitive.char.html#method.len_utf8"]' 'char::len_utf8'
 
 //! A [`char`] and its [`char::len_utf8`].