sim: ppc: unify igen filter modules

sim: ppc: unify igen filter modules

The common igen code was forked from the ppc long ago.  The filter
module is still pretty similar in API, so we can unfork them with
a little bit of effort.

The filter.c module is still here because of the unique it_is API.
The common igen code doesn't seem to have an equiv API as this only
operates on two strings and not an actual filter object, and it's
easy enough to leave behind to unfork the rest.