`core::mem::transmute` doesn't list `const` in the signature

`core::mem::transmute` and `std::mem::transmute` are **both** listed as const in 1.56 according to the "since" info on the right, but **only** the `std` version lists `const` in the actual signature.

core:
```
pub unsafe extern "rust-intrinsic" fn transmute<T, U>(e: T) -> U
```

std:
```
pub const unsafe extern "rust-intrinsic" fn transmute<T, U>(e: T) -> U
```