Eliminate the last MI difference in AT_* definitions (for powerpc).

Summary:
As a transition aide, implement an alternative elfN_freebsd_fixup which
is called for old powerpc binaries. Similarly, add a translation to rtld to
convert old values to new ones (as expected by a new rtld).

Translation of old<->new values is incomplete, but sufficient to allow an
installworld of a new userspace from an old one when a new kernel is running.

Test Plan:
Someone needs to see how a new kernel/rtld/libc works with an old
binary. If if works we can probalby ship this. If not we probalby need
some more compat bits.

Submitted by: brooks
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D20799