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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

Oh, I see a problem. From `targetSupportsPAuthRelocation`: "No released version of glibc supports PAuth relocations."

So basically, we need to disable PAuth relocations for glibc, but we also need to disable IFUNC for anything not glibc. What target is meant to be handled here?

Either way, even without this PR, #133533 looks like it has a problem, it results in a crash if the existing test is updated to use `aarch64-linux-gnu`.

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