riscv: Use elf_machine_rela_relative to handle R_RISCV_RELATIVE

Minor clean-up, we need to change this part in following patch, clean this up
to prevent we duplicated the change twice.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>