state of atomic support

Nobody has claimed this yet.

There are some patches for atomic support in the backend, but there are still some parts missing to build a rust memory allocator or rust's libstd.

```sh
LLVM ERROR: Cannot select: t32: i32,ch = AtomicCmpSwap t12, t4, t39, t26
t4: i32,ch = load t0, FrameIndex:i32<9>, undef:i32
t1: i32 = FrameIndex<9>
t3: i32 = undef
t39: i32,ch = load t0, FrameIndex:i32<8>, undef:i32
t5: i32 = FrameIndex<8>
t3: i32 = undef
t26: i32,ch = load t0, FrameIndex:i32<7>, undef:i32
t7: i32 = FrameIndex<7>
t3: i32 = undef
In function: _ZN4core4sync6atomic23atomic_compare_exchange17h1807facac1d33089E
```

```sh
LLVM ERROR: Cannot select: t22: i32,ch = AtomicCmpSwap t11, t8, Constant:i32<2>, Constant:i32<0>
t8: i32 = add nuw t2, Constant:i32<24>
t2: i32,ch = CopyFromReg t0, Register:i32 %0
t1: i32 = Register %0
t7: i32 = Constant<24>
t12: i32 = Constant<2>
t4: i32 = Constant<0>
In function: _ZN3std6thread4park17h5b16deb9482d0a5bE
```

Start by reproducing the two reported LLVM ERROR cases involving AtomicCmpSwap, including the failures in Rust's atomic_compare_exchange and std::thread::park. Trace the missing atomic backend support exposed by those cases; done means these operations compile without the reported instruction-selection errors.

Written by the indexing model from the issue text.