sim: ppc: switch to common builds for callback objects

sim: ppc: switch to common builds for callback objects

We don't need to build this anymore ourselves since the common build
includes it and produces the same object code.  We also need to pull
in the split constant modules after the refactoring and pulling them
out of nltvals.def & targ-map.o.  This doesn't matter for the sim
directly, but does for gdb and other users of libsim.

We also delete some conditional source tree logic since we already
require this be the "new" combined tree with a ../common/ dir.  This
has been the case for decades at this point.