[ELF] Write R_RISCV_IRELATIVE addends with -z rel

I found this missing case with the new --check-dynamic-relocation flag
while running the lld tests with --apply-dynamic-relocs enabled by default.

This is the same as D101452 just for RISC-V

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D101454