[llvm-ar][test] Unsupport error-opening-directory.test on FreeBSD

Differential Revision: https://reviews.llvm.org/D82786
diff --git a/llvm/test/tools/llvm-ar/error-opening-directory.test b/llvm/test/tools/llvm-ar/error-opening-directory.test
index 10cee214..1f19da7 100644
--- a/llvm/test/tools/llvm-ar/error-opening-directory.test
+++ b/llvm/test/tools/llvm-ar/error-opening-directory.test
@@ -1,3 +1,7 @@
+## Unsupported on FreeBSD as FreeBSD 12 and earlier allow reading directories
+## by default.
+# UNSUPPORTED: system-freebsd
+
 # RUN: rm -rf %t && mkdir -p %t
 
 ## Passing in a directory: