sim: ppc: move spreg.[ch] files to the source tree

sim: ppc: move spreg.[ch] files to the source tree

Simplify the build by moving the generation of these files from
build-time (via dgen.c that we have to compile & execute on the
build system) to maintainer/release mode (via spreg-gen.py that
we only ever execute when the spreg table actually changes).  It
speeds up the build process and makes it easier for us to reason
about & review changes to the code generator.

The tool is renamed from "dgen" because it's hardcoded to only
generated spreg files.  It isn't a generalized tool for creating
lookup tables.