[llvm] [llvm-exegesis][AArch64] Check for PAC keys before disabling them (PR #138643)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

The linux PAC launch path was explained to me on discord and my concerns were cleared up, I can't speak for the PR code itself, my concerns were with how the actual process launch was being handled structurally.

https://github.com/llvm/llvm-project/pull/138643