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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> Several other libcs support IFUNC, musl is the exception.

I do not think musl is alone in not supporting it, I see no support for it in uclibc-ng either.

> Can we make it so that `isMusl()` or some other predicate returns true for `pauthtest`?

I think that would be wrong, `pauthtest` does not imply any particular libc.

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