[PowerPC] Make new auxv format default

Assume ELF images without OSREL use the new auxv format.

This is specially important for rtld, that is not tagged. Using
direct exec mode with new (ELFv2) binaries that expect the new auxv
format would result in crashes otherwise.

Unfortunately, this may break direct exec'ing old binaries,
but it seems better to correctly support new binaries by default,
considering the transition to ELFv2 happened quite some time
ago. If needed, a sysctl may be added to allow old auxv format to
be used when OSREL is not found.

Reviewed by: bdragon
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D25651