kldxref: handle R_RISCV_64 relocation

These are emitted in at least two kmods, and kldxref prints a warning.
While here, remove the unneeded local variable 'val'.

Reviewed by: jrtc27, imp, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37038