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 dgen.py that we
only ever execute when the spreg table actually changes).  This
speeds up the build process and makes it easier for us to reason
about & review changes to the code generator.