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

[David Green via cfe-commits]

https://github.com/davemgreen commented:

The part that alters -mcpu=grace sounds good. I believe that can be committed separately and isn't dependant on the rest. The part that alters Host.cpp is related to -mcpu=native and could be better included of the other patch, so that -mcpu=native works correctly.

(I think how it would best work is to use hpcaps on linux and trust all the feaures we have, but that is a larger change).

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