split sys0.ji into a separate build rule

(approximately reverts 8044eb16351dcc66bd952c684b42069585944489 Makefile changes)

I think this gives us better visibility into partial build failures and simplifies the build rules somewhat. It also provides a backup sys.ji file

@JeffBezanson is this ok? I know it's been a while since you worked to eliminated sys0.ji :)