[llvm] [llvm-exegesis][AArch64] Disable pauth and ldgm as unsupported instructions (PR #132346)

[David Green via llvm-commits]

davemgreen wrote:

According to https://docs.kernel.org/arch/arm64/pointer-authentication.html, the authentication can be disable with something like 
```
prctl(PR_PAC_SET_ENABLED_KEYS,
      PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY | PR_PAC_APDBKEY,
      0, 0, 0);
```

On the systems I tried it on (a couple of neoverse-v2 machines), it allowed the instructions to execute and give sensible latency/throughputs. There is a chance it would make them run at a different speed, but that doesn't appear to be the case on the system I tried. That might be a better alternative than preventing any timing of the instructions.

https://github.com/llvm/llvm-project/pull/132346