Rollup merge of #66705 - pitdicker:atomic_mut_ptr, r=KodrAus

Atomic as_mut_ptr

I encountered the following pattern a few times: In Rust we use some atomic type like `AtomicI32`, and an FFI interface exposes this as `*mut i32` (or some similar `libc` type).

It was not obvious to me if a just transmuting a pointer to the atomic was acceptable, or if this should use a cast that goes through an `UnsafeCell`. See https://github.com/rust-lang/rust/issues/66136#issuecomment-557802477

Transmuting the pointer directly:
```rust
let atomic = AtomicI32::new(1);
let ptr = &atomic as *const AtomicI32 as *mut i32;
unsafe {
    ffi(ptr);
}
```

A dance with `UnsafeCell`:
```rust
let atomic = AtomicI32::new(1);
unsafe {
    let ptr = (&*(&atomic as *const AtomicI32 as *const UnsafeCell<i32>)).get();
    ffi(ptr);
}
```

Maybe in the end both ways could be valid. But why not expose a direct method to get a pointer from the standard library?

An `as_mut_ptr` method on atomics can be safe, because only the use of the resulting pointer is where things can get unsafe. I documented its use for FFI, and "Doing non-atomic reads and writes on the resulting integer can be a data race."

The standard library could make use this method in a few places in the WASM module.

cc @RalfJung as you answered my original question.

src/libstd/lib.rs

@@ -234,6 +234,7 @@
 #![feature(allocator_internals)]
 #![feature(allow_internal_unsafe)]
 #![feature(allow_internal_unstable)]
+#![feature(atomic_mut_ptr)]
 #![feature(arbitrary_self_types)]
 #![feature(array_error_internals)]
 #![feature(asm)]