Fix link flags with tracing. (#3531)

Closes 3392
diff --git a/build_tools/cmake/iree_copts.cmake b/build_tools/cmake/iree_copts.cmake
index 9d99d9b..74a29c4 100644
--- a/build_tools/cmake/iree_copts.cmake
+++ b/build_tools/cmake/iree_copts.cmake
@@ -39,7 +39,7 @@
 )
 
 if(${IREE_ENABLE_RUNTIME_TRACING})
-  set (CMAKE_EXE_LINKER_FLAGS -ldl)
+  string (APPEND CMAKE_EXE_LINKER_FLAGS -ldl)
 endif()
 
 iree_select_compiler_opts(IREE_DEFAULT_COPTS