atomic.fence is missing memory order immediate

[eqrion]

See binary format in [1]. There's a reserved single byte for a memory order argument for atomic.fence. It's not specifiable in the text format.

[1] https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md

[alexcrichton]

Oh dear, this is what happens when 99% of your tests are against another tool (in this case wabt) and then this implements more features than that tool! Thanks for the report though!

[eqrion]

Thanks for the quick fix!