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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

Updated accordingly, does that look okay? I'm a little bit concerned that a global fix like `isOSGlibc()` shouldn't be included in a commit that's meant to be limited to AArch64.

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