Support building just a sysroot, not a toolchain

[alexcrichton]

Right now this is sort of supported with the default CMake invocation of this repository but as pointed out in this comment and the discussion in that thread this doesn't fully work because compiler-rt is copied into the host toolchain. That's generally not a great thing to do.

Fixing this would require one of two things that I can think of:

1. Support not building compiler-rt. How exactly the host toolchain gets compiler-rt, however, I'm not certain.
2. Support the host compiler using the just-built compiler-rt. It sounds like -resource-dir is the way to go here according to the discussion in Fix determination of wasi-sdk version when built in a subdirectory #443 but I don't know how to communicate that or how best to document that (e.g. what is the exact structure of -resource-dir? What to do with target-specific headers? etc)