[llvm] github: Add llvm:mc label for generic MC interface (PR #153737)

[Fangrui Song via llvm-commits]

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@@ -621,6 +618,10 @@ llvm:adt:
 llvm:support:
   - llvm/**/Support/**
 
+# Skip llvm/test/MC, which includes target-specific directories
+llvm:mc:
+  - llvm/lib/MC/**
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MaskRay wrote:

Thanks for the suggestion! Done

https://github.com/llvm/llvm-project/pull/153737