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

[Lu Weining via llvm-commits]

SixWeining wrote:

> Is this the correct way to get the four parts of PCALA64?
> 
> ```
> DELTA = DEST - PC[63:12][000]
> DELTA = DELTA + (DELTA[11] << 12)
> DELTA = DELTA + (DELTA[31] << 32)
> DELTA = DELTA - (DELTA[11] << 32)
> 
> R_LARCH_PCALA_HI20   = DELTA[31:12]
> R_LARCH_PCALA_LO20   = DELTA[11:00]
> R_LARCH_PCALA64_LO20 = DELTA[51:32]
> R_LARCH_PCALA64_HI12 = DELTA[63:52]
> ```
> 
> This is a simple validator, let me test it with boundary data: https://gist.github.com/heiher/bd7398397f3cb5598dce35cbc04d0075

Seems this is what ld and mold do? Like bfd/elfnn-loongarch.c (`RELOCATE_CALC_PC64_HI32`).

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