Move `std::sys_common::alloc` to new module `std::sys::common`

https://github.com/rust-lang/rust/blob/6b56603e35b39c9f6cc76782330e5e415f9e43d5/library/std/src/sys_common/mod.rs#L7-L13

It was my impression that the goal for `std::sys` has changed from extracting it into a separate crate to making std work with features. However the fact remains that there is a lot of interdependence between `sys` and `sys_common`, this is because `sys_common` contains two types of code:

- abstractions over the different platform implementations in `std::sys` (for example [`std::sys_common::mutex`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys_common/mutex.rs))
- code shared between platforms (for example [`std::sys_common::alloc`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys_common/alloc.rs))

This PR attempts to address this by adding a new module `common` to `std::sys` which will contain code shared between platforms, `alloc.rs` in this case but more can be moved over in the future.

Move `std::sys_common::alloc` to new module `std::sys::common`

https://github.com/rust-lang/rust/blob/6b56603e35b39c9f6cc76782330e5e415f9e43d5/library/std/src/sys_common/mod.rs#L7-L13

It was my impression that the goal for `std::sys` has changed from extracting it into a separate crate to making std work with features. However the fact remains that there is a lot of interdependence between `sys` and `sys_common`, this is because `sys_common` contains two types of code:

- abstractions over the different platform implementations in `std::sys` (for example [`std::sys_common::mutex`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys_common/mutex.rs))
- code shared between platforms (for example [`std::sys_common::alloc`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys_common/alloc.rs))

This PR attempts to address this by adding a new module `common` to `std::sys` which will contain code shared between platforms, `alloc.rs` in this case but more can be moved over in the future.

Rollup of 7 pull requests

Successful merges:

 - rust-lang#82492 (Move `std::sys_common::alloc` to new module `std::sys::common`)
 - rust-lang#84177 (Fix join_paths error display.)
 - rust-lang#84185 (add more pat2021 tests)
 - rust-lang#84191 (Update books)
 - rust-lang#84192 (Fix typos in rustc_codegen_ssa/src/back/write.rs.)
 - rust-lang#84196 (:arrow_up: rust-analyzer)
 - rust-lang#84201 (rustdoc: Note that forbidding anchors in links to primitives is a bug)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup