mark aro sources as vendored

Author: Vexu

.gitattributes

@@ -12,4 +12,4 @@ lib/libcxx/** linguist-vendored
 lib/libcxxabi/** linguist-vendored
 lib/libunwind/** linguist-vendored
 lib/tsan/** linguist-vendored
-deps/** linguist-vendored
+lib/compiler/aro/** linguist-vendored