Update llvm.yml to install AArch64 cross compilers

.github/workflows/llvm.yml

@@ -27,6 +27,10 @@ jobs:
       - uses: actions/checkout@v4
       - uses: lukka/get-cmake@latest
 
+      - name: Install cross compilers
+        if: startsWith(matrix.config.os, 'ubuntu') && startsWith(matrix.config.platform, 'arm64')
+        run: sudo apt install -y g++-aarch64-linux-gnu gcc-aarch64-linux-gnu
+
       - name: Clone LLVM
         shell: bash
         run: build/build.sh clone_llvm