[RISCV] Mark QC Relocations as Relaxable (#142794)
Some of the QC relocations are relaxable, in particular there are
relaxations defined for:
- `R_RISCV_QC_E_JUMP_PLT`
- `R_RISCV_QC_E_32`
- `R_RISCV_QC_ABS20_U`
This change ensures that llvm-mc correctly emits R_RISCV_RELAX
relocations for the relevant fixups.
