Auto merge of #155521 - Urgau:runtime-symbols, r=davidtwco

Add lint against invalid runtime symbol definitions



This PR adds a deny-by-default lint againts invalid runtime symbol definitions, those runtime symbols are assumed and used by `core`[^1] and `rustc` with a specific definition.

We have had multiple reports of users tripping over `std` symbols (addressed in a future PR):
 - [Why does `#[no_mangle] fn open() {}` make `cargo t` hang?](https://users.rust-lang.org/t/why-does-no-mangle-fn-open-make-cargo-t-hang/103423)
 - [Pointer becomes misaligned in test with `no_mangle`](https://users.rust-lang.org/t/pointer-becomes-misaligned-in-test-with-no-mangle/126580)

This PR is a second attempt after https://github.com/rust-lang/rust/pull/146505, where T-lang had [some reservations](https://github.com/rust-lang/rust/pull/146505#issuecomment-3492657109) about a blanket lint that does not check the signature, which is now done with this PR, and about linting of `std` runtime symbols when std is not linked, which this PR omits by not including any std runtime symbols (for now). 

## `invalid_runtime_symbol_definitions`

*(deny-by-default)* 

The `invalid_runtime_symbol_definitions` lint checks the signature of items whose symbol name is a runtime symbols expected by `core` differs significantly from the expected signature (like mismatch ABI, mismatch C variadics, mismatch argument count, missing return type, ...).

### Example

```rust,compile_fail
#[unsafe(no_mangle)]
pub fn memcmp() {} // invalid definition of the `memcmp` runtime symbol
```

```text
error: invalid definition of the runtime `memcmp` symbol used by the standard library
 --> a.rs:2:1
  |
4 | fn memcmp() {}
  | ^^^^^^^^^^^
  |
  = note: expected `unsafe extern "C" fn(*const c_void, *const c_void, usize) -> i32`
          found    `fn()`
  = help: either fix the signature or remove any attributes like `#[unsafe(no_mangle)]`, `#[unsafe(export_name = "memcmp")]`, or `#[link_name = "memcmp"]`
  = note: `#[deny(invalid_runtime_symbol_definitions)]` on by default
```

### Explanation

Up-most care is required when defining runtime symbols assumed and used by the standard library. They must follow the C specification, not use any standard-library facility or undefined behavior may occur.

The symbols currently checked are `memcpy`, `memmove`, `memset`, `memcmp`, `bcmp` and `strlen`.

## `suspicious_runtime_symbol_definitions`

(warn-by-default, asked in [this comment](https://github.com/rust-lang/rust/pull/155521#issuecomment-4696528297))

The `suspicious_runtime_symbol_definitions` lint checks the signature of items whose symbol name is a runtime symbol expected by `core`.

### Example

```rust
#[unsafe(no_mangle)]
pub extern "C" fn strlen(ptr: *mut f32) -> usize { 0 }
// suspicious definition of the `strlen` function
// `ptr` should be `*const std::ffi::c_char`
```

### Explanation

Up-most care is required when defining runtime symbols assumed and used by the standard library. They must follow the C specification, not use any standard-library facility or undefined behavior may occur.

[^1]: https://doc.rust-lang.org/core/index.html#how-to-use-the-core-library