SCons: Make the ISA parser a source for its output files like the comments say.

SCons: Make the ISA parser a source for its output files like the comments say.

There was a change a while ago that refactored some scons stuff which got rid
of cpu_models.py but also accidentally got rid of the ISA parser as a source
for its target files. That meant that changes which affected the parser
wouldn't cause a rebuild unless they also changed one of the description
files. This change fixes that.