kldxref: add R_PPC_ADDR32 handler

Implements R_PPC_ADDR32 handler for kldxref, addressing
"kldxref: unhandled relocation type 1" when running
"kldxref /boot/kernel" on powerpc 32 bits.

Reviewed by: luporl
MFC after: 2 days
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D33745

(cherry picked from commit 79f5d19890c3a703b3808af2dffe9d181a2e1696)