Missing 64-bit AtomicCmpxchg causes `__sync_fetch_and_nand` to fail under wasm2js

[sbc100]

Usage of __sync_fetch_and_nand with 64-bit operands in emscripten causes the following error:

wasm2js: /usr/local/google/home/sbc/dev/wasm/binaryen/src/passes/I64ToI32Lowering.cpp:471: void wasm::I64ToI32Lowering::visitAtomicCmpxchg(wasm::AtomicCmpxchg *): Assertion `curr->type != Type::i64 && "64-bit AtomicCmpxchg not implemented"' failed.

I think this is probably super rare and not a huge deal, but creating this issue so I can link to it in the test code.

[sbc100]

@kripken should we care about this?

[kripken]

I don't think I heard of this affecting real-world code, so it seems like a very low priority to me.