Web Assembly Atomic fence support in `atomic::fence`

[poliorcetics]

The function pub fence(order: Ordering) in libcore/sync/atomic.rs does nothing when compiled for Web Assembly.

This stem from an old issue in web assembly #WebAssembly/tool-conventions/issues/59 and LLVM crashing (at the time) with it.

This has since been fixed, notably for LLVM, maybe we can remove the #[cfg(...)] and comment about it ?