Fix the powerpc copyout fixup from r356113

Summary:
r356113 used an older patch, which predated the
freebsd_copyout_auxargs() addition. Fix this by using a private
powerpc_copyout_auxargs() instead, and keep it private to powerpc, not in MI
files.

Reviewed by: kib, bdragon
Differential Revision: https://reviews.freebsd.org/D22935