Just completed my article, Cross compilation for Raspberry from Sierra, about exactly this thing. Under Mac OS Sierra though. But for clang it matters a little.
In this article I have shared experience (which costed me some days of suffering). Hope it will be useful.
In short, all you have to do is to specify rootfs and target triple:
I followed this article and it seems to work fine for trivial programs (console/string/math stuff, not many optimization flags). I used ver 1.19 of crosstool-ng on a x86-64 laptop running ZenWalk 7.0
There also seems to be this post over at SO, which might be helpful (haven't fully read thru it yet).