[PowerPC] libc backwards compatibility shim for auxv change

As part of the FreeBSD powerpc* flag day (1300070), the auxv numbering was
changed to match every other platform.

See D20799 for more details on that change.

While the kernel and rtld were adapted, libc was not, so old dynamic
binaries broke for reasons other than the ABI change on powerpc64.

Since it's possible to support nearly everything regarding old binaries by
adding compatibility code to libc (as besides rtld, it is the main point
where auxv is digested), we might as well provide compatibility code.

The only unhandled case remaining should be "new format libraries that call
elf_aux_info() which are dynamically linked to by old-format binaries",
which should be quite rare.

Reviewed by: jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D23096