Register frama-clang known attributes

The following attributes are registered to Frama-C's list of known
attributes:
- fc_implicit
- fc_pure_template_decl

tests/basic/oracle/printer.res.c

@@ -27,7 +27,7 @@ struct _ZN1AE1B {
 struct _Z28_frama_c_rtti_name_info_node _ZN1A1BE23_frama_c_rtti_name_info;
 
 /*@ requires \valid_read(this); */
-void _ZN1A1BEC1(struct _ZN1AE1B const *this __attribute__((____fc_initialized_object__)))
+void _ZN1A1BEC1(struct _ZN1AE1B const *this __attribute__((__fc_initialized_object__)))
 {
   this->x = 42;
   return;