[lld] [lld][ELF] Fix a corner case of elf::getLoongArchPageDelta (PR #71907)

[Lu Weining via llvm-commits]

SixWeining wrote:

> ```assembly
> .L1:
>   pcalau12i $a1, 0        # R_LARCH_PCALA_HI20
>   addi.d    $a0, $zero, A # R_LARCH_PCALA_LO12
>   lu32i.d   $a0, B        # R_LARCH_PCALA64_LO20
>   lu52i.d   $a0, $a0, C   # R_LARCH_PCALA64_HI12
>   add.d     $a0, $a0, $a1
> ```

Is it possible to use the `r_addend` of `Elf64_Rela`  instead of insn imm bits to store the adjusting bits?

For example:

### Before relocation

The addend should be written by linker but not assembler because assembler doesn't know the page delta  of current instruction and `pcalau12i`. (Same as the imm bits approach?)

```
0000000000000000 <_start>:
   0:   1a000004        pcalau12i       $a0, 0
                        0: R_LARCH_PCALA_HI20   foo
   4:   02c00001        addi.d          $ra, $zero, 0
                        4: R_LARCH_PCALA_LO12   foo
   8:   16000001        lu32i.d         $ra, 0
                        8: R_LARCH_PCALA64_LO20 foo+0x10000000
   c:   03000021        lu52i.d         $ra, $ra, 0
                        c: R_LARCH_PCALA64_HI12 foo+0x10000000000000
  10:   00109021        add.d           $ra, $ra, $a0
  14:   4c000021        jirl            $ra, $ra, 0
```

### After link

```
0000000080000ffc <_start>:
    80000ffc:   1b000004        pcalau12i       $a0, -524288(0x80000)
    80001000:   02c00001        addi.d          $ra, $zero, 0
    80001004:   16000001        lu32i.d         $ra, 0
    80001008:   03000021        lu52i.d         $ra, $ra, 0
    8000100c:   00109021        add.d           $ra, $ra, $a0
```

https://github.com/llvm/llvm-project/pull/71907