Fix kldxref on PowerPC64

When using kldxref on kernel modules built with clang8 + lld8,
kldxref would be unable to find the modules metadata information,
because PowerPC64 was using the ef_nop.c implementation of
ef_reloc().

When GNU LD was used, it was also relocating the metadata section of
the .ko file. LLD does not do this, but only generate dynamic
relocations for it. With minor changes, ef_powerpc.c can now work
for PowerPC64 too.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D19370