make_conf: support passing a Herd C model

By default, make_conf doesn't tell Herd to use a particular C model, ie the c11_lahav.cat in the ACT directory. This means that there's a mismatch between the Herd model and the Memalloy one, causing weird behaviour (mainly, I think?, in the form of Memalloy generating racy code and Herd rejecting it as UB? though this almost seems like it should be the case, huh.)

It's probably best if the user/Dockerfile can help make_conf out here.