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

As a member of github.com/orgs/llvm/teams/pr-subscribers-llvm-mc , I was
not notified about PR llvm#149935.

This commit introduces the `llvm:mc` label to cover the generic MC
interface, excluding target-specific MCTargetDesc files.

- Rename the `mc` label to `llvm:mc` for consistency with other LLVM
subdirectory labels.
- Exclude `llvm/test/MC` from the label scope, as it contains many
target-specific directories.

Admin: please change the name of
https://github.com/orgs/llvm/teams/pr-subscribers-llvm-mc
to "pr-subscribers-llvm:mc", similar to pr-subscribers-llvm:ir

Author: MaskRay

.github/new-prs-labeler.yml

@@ -90,9 +90,6 @@ LTO:
   - llvm/lib/Transforms/*/FunctionImport*
   - llvm/tools/gold/**
 
-mc:
-  - llvm/*/MC/**
-
 clang:driver:
   - clang/*/Driver/**
 
@@ -621,6 +618,12 @@ llvm:adt:
 llvm:support:
   - llvm/**/Support/**
 
+# Skip llvm/test/MC and llvm/unittests/MC, which includes target-specific directories.
+llvm:mc:
+  - llvm/include/llvm/MC/**
+  - llvm/lib/MC/**
+  - llvm/tools/llvm-mc/**
+
 llvm:transforms:
   - llvm/lib/Transforms/**
   - llvm/include/llvm/Transforms/**