[clang] [llvm] [AArch64] Disable FEAT_RNG on Neoverse V2. (PR #166387)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

This should be left on. People can use -mcpu=neoverse-v2+norng if they don't have it. Lots of features can be disabled for many different reasons. If Grace doesn't have it then maybe that should apply just to Grace, and we split the configs into two?

The mcpu=native change looks sensible. We should be doing that for all features AFAIU (and maybe getting them from hwcaps). 

https://github.com/llvm/llvm-project/pull/166387