sim: ppc: convert to bfd_endian

sim: ppc: convert to bfd_endian

Rather than re-invent endian defines, as well as maintain our own list
of OS & arch-specific includes, punt all that logic in favor of the bfd
ones already set up and maintained elsewhere.  We already rely on the
bfd library, so leveraging the endian aspect should be fine.

This was done for all the other ports years ago, so catch ppc up.