Fix rtld-elf compilation warning for powerpc64 ELFv2 ABI

Summary: reloc_jmpslot function parameter 'defobj' is not used when using ELFv2
ABI

Submitted by: alfredo.junior_eldorado.org.br
Reviewed By: kib, git_bdragon.rtk0.net, emaste, jhibbits
Differential Revision: https://reviews.freebsd.org/D18808