After my previous post I managed to build the compiler myself. It turns out that when Emscripten installs itself, it installs its own copy of clang and puts that in the path. CoreRT falls over when it tries to use v6 instead of v3.9, so the solution was to uninstall Emscripten.

Even then, after installing the compiler and getting all the tests to pass, I was still running into some issues. Basically, when I'm trying to link to the static library I built, it was throwing undefined references to things like CoreRT's RhpNewArray.

That, and the fact that the most recent version of the static library has ballooned to more than 30MB with only a couple of test functions, is leading me to abandon this approach. Maybe I'll come back to this when CoreRT is a bit more mature.