workflows/repo-lockdown: Ignore libcxx and related sub-directories

https://discourse.llvm.org/t/opening-up-prs-experimentally-for-a-subset-of-the-llvm-project/70375/11

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D150314

Author: tstellar

.github/workflows/repo-lockdown.yml

@@ -2,6 +2,11 @@ name: 'Repo Lockdown'
 on:
   pull_request_target:
     types: opened
+    paths-ignore:
+      - 'libcxx/**'
+      - 'libcxxabi/**'
+      - 'libunwind/**'
+      - 'runtimes/**'
 
 permissions:
   pull-requests: write