[PATCH] Implement emitLeading/TrailingFence in the ARM backend

[Robin Morisset]

Thanks for the review,

Indeed, this patch is used in D4961. I can fairly easily merge the two patches if you want, I'm still struggling to find the right way of cutting patches (too big and it is hard to review, too small and it is temporarily dead code like here).
I will fix the other things you point out ASAP.

http://reviews.llvm.org/D4960