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

[Anton Korobeynikov via llvm-commits]

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asl wrote:

```suggestion
  if (TT.getEnvironment() != Triple::PAuthTest && 
      (TT.isOSGlibc() || TT.isMusl()))
```

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