[lld] [PAC][lld] Fix reloc against adrp imm in PAC PLT header (PR #113429)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM.

This was discussed in Discord under https://discord.com/channels/636084430946959380/1133112394701348895/1298121731428585564 

I think the existing incorrect value of 8 is always going to round down to the same page [1] as 4 by getAArch64Page, but it is worth getting this right in the source code.

[1] PLT is 16-byte aligned. I don't think there's a placement of the PLT such that PLT + 4 or PLT +8 can cross a 4KiB page boundary.

https://github.com/llvm/llvm-project/pull/113429