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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> Could you please update the PR description adding info about the error introduced in https://github.com/llvm/llvm-project/pull/133533 which is fixed by this PR?

I do mention already that this fixes aarch64-linux-pauthtest, but that particular error can still occur for other targets, because that is not specific to aarch64-linux-pauthtest, but to -fptrauth-elf-got. I am working on a separate PR fixing that.

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