[Object][RISCV] Fix R_RISCV_SET6 and R_RISCV_SUB6 relocations resolution

Summary: These relocations had a suspicious resolution logic, given their name.
This patch makes the resolution match the LLD one, which makes more sense.

Reviewers: asb, lenary, HsiangKai, jrtc27
Reviewed By: HsiangKai
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70396