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

[Tomas Matheson via llvm-commits]

https://github.com/tmatheson-arm commented:

There is an existing mechanism for processor aliases in `TargetParser`:
```cpp
inline constexpr Alias CpuAliases[] = {{"cobalt-100", "neoverse-n2"},
                                       {"grace", "neoverse-v2"}};
```
Please could you either incorporate these CPUs into what you're doing here, or use the existing mechanism?

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