Rather than include this for some targets, set it up so we can build it
all the time via the common code. This makes it easier for targets to
opt into it when they're ready, increases build coverage, and allows us
to centralize much of the logic.
We also get to delete tconfig.h from two more targets -- they were
setting WITH_DEVICES to 0 which has the same behavior as not defining
it at all.
While the SIM_HAVE_MODEL knob is gone, we now have WITH_MODEL_P, but it
is only used by the common sim-model code. We use it to declare dummy
model lists when the arch hasn't created its own.
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