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

[via llvm-commits]

heiher 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`).

As jinyang said, this way relies on the PC of `pcalau12i`, which is difficult to do when there is instruction scheduling.

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