[llvm] [llvm][AArch64][TableGen] Create a ProcessorAlias record. NFC (PR #96249)

[David Green via llvm-commits]

davemgreen wrote:

We have at the moment:
```
inline constexpr Alias CpuAliases[] = {{"cobalt-100", "neoverse-n2"},
                                       {"grace", "neoverse-v2"}};
```
Can all the cpu aliases work in the same way? Or do you want them to work different, where they are not just frontend aliases?

https://github.com/llvm/llvm-project/pull/96249