implement fence for wasm32

[hellow554]

rust/src/libcore/sync/atomic.rs

Lines 2422 to 2440 in 6576f4b

	#[cfg_attr(target_arch = "wasm32", allow(unused_variables))]
	pub fn fence(order: Ordering) {
	// On wasm32 it looks like fences aren't implemented in LLVM yet in that
	// they will cause LLVM to abort. The wasm instruction set doesn't have
	// fences right now. There's discussion online about the best way for tools
	// to conventionally implement fences at
	// https://github.com/WebAssembly/tool-conventions/issues/59. We should
	// follow that discussion and implement a solution when one comes about!
	#[cfg(not(target_arch = "wasm32"))]
	unsafe {
	match order {
	Acquire => intrinsics::atomic_fence_acq(),
	Release => intrinsics::atomic_fence_rel(),
	AcqRel => intrinsics::atomic_fence_acqrel(),
	SeqCst => intrinsics::atomic_fence(),
	Relaxed => panic!("there is no such thing as a relaxed fence"),
	}
	}
	}

The comment says that the issue should be watched, but it has been closed for 2 months now with an implementation for wasm32 at llvm/llvm-project@d85fd5a

The branch rust uses doesn't include that commit yet (llvm/llvm-project@d85fd5a), but it's available.

Maybe llvm can be updated and the config attributes removed?