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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

I took out the removal of `targetSupportsPAuthRelocation` from this PR. It was a good suggestion to look at but in retrospect I don't think it's right, unless I am misunderstanding. For targets that support some PAuth relocations directly, and also support IFUNCs, we should still emit those PAuth relocations directly, that is what that `targetSupportsPAuthRelocation` check achieved before, I don't see how we can do that another way.

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