Enable -Wtrampolines by default.

--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -712,7 +712,7 @@ Common Var(warn_system_headers) Warning
 Do not suppress warnings from system headers.
 
 Wtrampolines
-Common Var(warn_trampolines) Warning
+Common Var(warn_trampolines) Init(1) Warning
 Warn whenever a trampoline is generated.
 
 Wtype-limits