Use IBT by default on x86-64 BSD

Author: xavierleroy

driver/Clflags.ml

@@ -38,7 +38,10 @@ let option_mthumb = ref (Configuration.model = "armv7m")
 let option_Osize = ref false
 let option_finline = ref true
 let option_finline_functions_called_once = ref true
-let option_fcf_protection = ref false
+let option_fcf_protection = ref
+  (match Configuration.arch, Configuration.model, Configuration.system with
+    | "x86", "64", "bsd" -> true
+    | _ -> false)
 let option_dprepro = ref false
 let option_dparse = ref false
 let option_dcmedium = ref false