[llvm-branch-commits] [NFC][ELF][AArch64] Replace memtag hack with less-confusing code (PR #171182)

[Jessica Clarke via llvm-branch-commits]

https://github.com/jrtc27 created https://github.com/llvm/llvm-project/pull/171182

The current implementation in addRelativeReloc makes it look like we're
writing the symbol's VA + addend to the section, because that's what the
given relocation will evaluate to, but we're supposed to be writing the
negated original addend (since the relative relocation's addend will be
the sum of the symbol's VA and the original addend). This only works
because deep down in AArch64::relocate we throw away the computed value
and peek back inside the relocation to extract the addend and negate it.

Do this properly by having a relocation that evaluates to the right
value instead.