flambda-backend: Mark symbols as having visibility STV_PROTECTED (#2221)

* Mark symbols as having visibility STV_PROTECTED

* protected only works on linux systems

* typo

* add directive for protected

* Guard against a flag

* Code review

Author: poechsel

driver/main_args.ml

@@ -296,7 +296,6 @@ let mk_no_probes f =
     "-no-probes", Arg.Unit f, " Ignore [%%probe ..]"
 ;;
 
-
 let mk_labels f =
   "-labels", Arg.Unit f, " Use commuting label mode"