stand/reloc_elf: Handle relative relocations for arm{,64} and riscv

Extend the powerpc relative relocation handling from r240782 to a
handful of other architectures. This is needed to properly read
dependency information from kernel modules.

Reviewed by: jhb
Approved by: scottl (implicit)
MFC after: 1 week
Sponsored by: Ampere Computing, Inc.
Differential Revision: https://reviews.freebsd.org/D26365