[LoongArch] Emit R_LARCH_RELAX when expanding some LoadAddress (#72961)

Emit relax relocs when expand non-large la.pcrel and non-large la.got on
llvm-mc stage, which like what does on GAS.
1, la.pcrel -> PCALA_HI20 + RELAX + PCALA_LO12 + RELAX
2, la.got -> GOT_PC_HI20 + RELAX + GOT_PC_LO12 + RELAX