[PATCH] Implement emitLeading/TrailingFence in the ARM backend

[Tim Northover]

> I'd remove the comment but I think the benchmark is still good to do: different ARM implementations perform very differently. I assume Tim measured on Apple hardware? We can try out the ones we have on hand here to make sure the results are the same.

Yep, it was just a very quick trial on an iPad I had lying around.
It's definitely worth checking on other hardware (and in more
realistic situations than a tight DMB/ISB loop) before dismissing the
idea completely.

The new patch looks fine to me, though. Those are questions for another day.

Cheers.

Tim.