[clang] [llvm] [AArch64][TargetParser] Add mcpu alias for Microsoft Azure Cobalt 100. (PR #79614)

[David Spickett via llvm-commits]

https://github.com/DavidSpickett approved this pull request.

```
/var/lib/buildkite-agent/builds/linux-56-59b8f5d88-r6v66-1/llvm-project/github-pull-requests/llvm/unittests/TargetParser/TargetParserTest.cpp:1662: Failure
Expected equality of these values:
  List.size()
    Which is: 68
  NumAArch64CPUArchs
    Which is: 67
```
Needs fixing, but otherwise LGTM.

https://github.com/llvm/llvm-project/pull/79614