kldxref: add R_PPC64_ADDR64 handler

Implements R_PPC64_ADDR64 handler for kldxref, addressing
"kldxref: unhandled relocation type 38" when running
"kldxref /boot/kernel"

Reviewed by: jhibbits
MFC after: 1 week
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D33615

(cherry picked from commit c3d9c63329a8b70a0a3b139c7819ace13aab6716)