Macros exported by compiler_builtins should not be in the prelude

[Amanieu]

compiler_builtins exports some macros for use by its tests, but rustc seems to make these available to any code as if they were part of the prelude. While compiler_builtins should be fixed to not export these, it is also strange that the prelude includes these macros.

// Compile with: rustc foo.rs --target aarch64-unknown-linux-gnu --crate-type lib
#![no_std]

foreach_cas16!();

error: unexpected end of macro invocation
 --> src/lib.rs:3:1
  |
3 | foreach_cas16!();
  | ^^^^^^^^^^^^^^^^ missing tokens in macro arguments
  |
note: while trying to match meta-variable `$r#macro:path`
 --> /home/amanieu/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/obj/build/x86_64-unknown-linux-gnu/stage1-std/aarch64-unknown-linux-gnu/release/build/compiler_builtins-0862f16702efa101/out/outlined_atomics.rs:103:51

error: aborting due to previous error

[lukas-code]

The prelude for no_std crates currently looks like this:

#![feature(prelude_import)]
#![no_std]
#[prelude_import]
use core::prelude::rust_2021::*;
#[macro_use]
extern crate core;
#[macro_use]
extern crate compiler_builtins;

The #[macro_use] on compiler_builtins comes from here:

rust/compiler/rustc_builtin_macros/src/standard_library_imports.rs

Line 57 in 55e8df2

thin_vec![cx.attr_word(sym::macro_use, span)],

[Amanieu]

In #113557 I changed this to:

// No #[macro_use]
extern crate compiler_builtins as  _;

The API of compiler-builtins is not stable and should not be exposed in the prelude. It's only needed there because compiler_builtins is required at link-time.