Rollup merge of rust-lang#56808 - jrvidal:broken-links, r=kennytm

Fixes broken links

Just a few broken links.

Not sure what to do about this one: https://github.com/rust-lang/rust/blame/master/src/doc/unstable-book/src/language-features/plugin.md#L135 (regex macros were removed a while ago in rust-lang/regex@0375954).

src/doc/rustc/src/targets/built-in.md

@@ -6,5 +6,5 @@ the team is supporting directly.
 
 To see the list of built-in targets, you can run `rustc --print target-list`,
 or look at [the API
-docs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_back/target/#modules).
+docs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/index.html#modules).
 Each module there defines a builder for a particular target.

src/doc/rustc/src/targets/index.md

@@ -4,7 +4,7 @@
 architecture. The list of *targets* are the possible architectures that you can build for.
 
 To see all the options that you can set with a target, see the docs
-[here](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_back/target/struct.Target.html).
+[here](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.Target.html).
 
 To compile to a particular target, use the `--target` flag:
 

src/doc/unstable-book/src/language-features/unsized-locals.md

@@ -8,7 +8,7 @@ The tracking issue for this feature is: [#48055]
 
 This implements [RFC1909]. When turned on, you can have unsized arguments and locals:
 
-[RFC1909]: https://github.com/rust-lang/rfcs/blob/master/text/1909-coercions.md
+[RFC1909]: https://github.com/rust-lang/rfcs/blob/master/text/1909-unsized-rvalues.md
 
 ```rust
 #![feature(unsized_locals)]