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

[Peter Collingbourne via llvm-commits]

pcc wrote:

Several other libcs support IFUNC, musl is the exception. Can we make it so that `isMusl()` or some other predicate returns true for `pauthtest`?

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