kldxref: handle R_RISCV_64 relocation

[mhorne]

Summary

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

For reference: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#relocations

Test Plan

Before:

# kldxref /boot/kernel
kldxref: unhandled relocation type 2
kldxref: unhandled relocation type 2
kldxref: unhandled relocation type 2
kldxref: unhandled relocation type 2
kldxref: unhandled relocation type 2

After:

# kldxref /boot/kernel