[PowerPC] [DTrace] Add ELFv2 support in libdtrace

PPC64 ELFv2 acts like a "normal" platform in that it no longer needs
function descriptors. So, ensure we are only enabling them on ELFv1.

Additionally, ELFv2 requires that the ELF header have a nonzero e_flags,
so ensure that the synthesized ELF header in dt_link.c is setting it.

Reviewed by: jhibbits, markj
Approved by: gnn
Differential Revision: https://reviews.freebsd.org/D22403