Use elf_relocaddr() when handling R_X86_64_RELATIVE relocations.

This is required for DPCPU and VNET data variable definitions to work when
KLDs are linked as DSOs. R_X86_64_RELATIVE relocations should not appear
in object files, so assert this in elf_relocaddr().

Reviewed by: kib
MFC after: 1 month
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21755