More reliable and complete build script

This builds QEMU with gcc, CIL (using CoroCheck) and CPC (with ucontext). It makes sure the latest version of QEMU, CIL and CPC are pulled before building and makes things as clean as possible before starting (use `DONTCLEAN=1 ./build_host.sh` to avoid rebuilding everything).

It does ***not*** call `git reset --hard` though, which lets you try tweaks in `cil`, `cpc` or `qemu` without committing them.