Support building of system image in binary builds

I'm going to wait for travis to turn green before I merge
:/ where was the bikeshedding in #8074 ? Does `getopt` let you do long args with a single dash? If not then the closest we would get would be `--march=...`. We could look into switching to whatever LLVM uses for command-line option parsing, but that's not a right-before-the-tag type change.