Add support for BTCFI

Index: backend/PrintAsmaux.ml
--- backend/PrintAsmaux.ml.orig
+++ backend/PrintAsmaux.ml
@@ -123,7 +123,7 @@ let elf_print_var_info oc name =
 (* Emit .cfi directives *)
 let cfi_startproc =
   if Configuration.asm_supports_cfi then
-    (fun oc -> fprintf oc "	.cfi_startproc\n")
+    (fun oc -> fprintf oc "	.cfi_startproc\n	 endbr64\n")
   else
     (fun _ -> ())
 
