LoongArchAsmBackend: Simplify relaxDwarfLineAddr and remove getRelocPairForSize
Instead of creating two separate fixups, create a single one. Leverage
LoongArchAsmBackend::addReloc to generate ADD/SUB relocation pairs.
In a future change MCFragment::setVarFixup may be restricted to a single
fixup.
Similar to c7500a2ec3baae1f0d7de0de94407d4bdb2e5d4d