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

[Anton Korobeynikov via llvm-commits]

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asl wrote:

The change above also implicitly changed the behavior here (as `isOSGlibc()` started to return `false` for `pauthtest`). In order to correctly represent the intended semantics we need to gate the libc check with environment check first as this also would protect us from potential future changes if / when `pautest` might imply a particular libc implementation.

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