Add PowerPC64 function descriptor support for dt_link.c

Summary:
PowerPC64 uses function descriptors in a section .opd, exporting the descriptors
to the symbol table. This adds support for these into dt_link.c so that dtrace
USDT probes can be compiled.

Test Plan:
Tested only on powerpc64. No regression testing has been performed, so I want
someone with x86 hardware to regression test this.

Tested on amd64 by markj

Reviewers: DTrace, markj

Reviewed By: DTrace, markj

Subscribers: markj

Differential Revision: https://reviews.freebsd.org/D1267

MFC after: 3 weeks