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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> So how about we make the logic like this: if the libc supports IFUNC (according to the list above), use IFUNC, otherwise use PAuth relocations. That's still not 100% correct because we will end up using PAuth relocations even when we don't know that the target supports them, but that was the status quo before so maybe that's fine.

Thanks, I'll update this PR accordingly.

I've posted the details about the crash that I noticed earlier on #133533, but I think we can handle that separately from this PR.

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