[lld][LoongArch] GOT indirection to PC relative optimization (#123743)

In LoongArch, we try GOT indirection to PC relative optimization in
normal or medium code model, whether or not with R_LARCH_RELAX
relocation.

From:
* pcalau12i $a0, %got_pc_hi20(sym_got)
* ld.w/d $a0, $a0, %got_pc_lo12(sym_got)

To:
* pcalau12i $a0, %pc_hi20(sym)
* addi.w/d $a0, $a0, %pc_lo12(sym)

If the original code sequence can be relaxed into a single instruction
`pcaddi`, this patch will not be taken (see
https://github.com/llvm/llvm-project/pull/123566).
The optimization related to GOT is split into two locations because the
`relax()` function is part of an iteration fixed-point algorithm. We
should minimize it to achieve better linker performance.
    
Note: Althouth the optimization has been performed, the GOT entries
still exists, similarly to AArch64. Eliminating the entries will
increase code complexity.

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