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

[Peter Collingbourne via llvm-commits]

pcc wrote:

I think we should do the simpler thing of only having a condition for IFUNC, especially given that no current target supports both, so the question of what we should do then is theoretical. If a target supports both and needs the slight code size improvement then we can consider adding code to prefer PAuth relocations in that case.

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