[llvm] [AArch64] Make IFUNC opt-in rather than opt-out. (PR #171648)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

About the `err-disc-elf.ll` -> `err-disc-gnu.ll` test update: that part of the test tests behaviour that's specific to IFUNC relocations, with PAuth relocations it crashes. Which is probably an issue, but also probably not something we need to fix in this specific PR?

https://github.com/llvm/llvm-project/pull/171648