sim: ppc: always compile in the sysv sem & shm device files

Move the stub logic to the device files themselves.  This makes the
configure & build logic more static which will make it easier to move
to the top-level build, and matches what we did with the common/ hw
tree already.

This also decouples the logic from the two -- in the past, you needed
both sem & shm in order to enable the device models, but now each one
is tied to its own independent knob.  Practically speaking, this will
probably not make a difference, but it simplifies the build a bit.