sim: ppc: drop support for std-config.h overrides

sim: ppc: drop support for std-config.h overrides

Only the ppc arch supports this kind of source file override logic.
All the others expose knobs via configure flags, and for some of
these, the ppc code does as well.  For others, it doesn't make sense
to ever change them.  Since it's unlikely anyone is using this, drop
it all to simplify the code (and to get us a little closer to the
common sim code).