elftoolchain: update the list of riscv relocations

A few have been added, a couple removed.

Reviewed by: jrtc27, imp, emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38175

(cherry picked from commit 537c166b767879398338370701fa60f67acec112)