Feature `compiler_builtins_lib` is mentioned in docs but has been removed

[tomaka]

The compiler_builtins_lib feature is still mentioned at several places in the documentation: https://github.com/rust-lang/rust/search?q=compiler_builtins_lib&unscoped_q=compiler_builtins_lib

However trying to use it in code leads to an error:

error[E0635]: unknown feature `compiler_builtins_lib`
  --> kernel/standalone/src/main.rs:22:12
   |
22 | #![feature(compiler_builtins_lib)]
   |            ^^^^^^^^^^^^^^^^^^^^^