riscv: fix relocation handling for R_RISCV_64

It requires the addend. In practice this doesn't seem to be a problem,
since relocations of this type are all with an addend of zero.
Obviously, we still want to handle this correctly if that ever changes.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37039