flambda-backend: Don't pass (unnecessary?) -L flags to gcc -shared from opttoplevel (#1363)

Author: ccasin

utils/ccomp.ml

@@ -168,7 +168,7 @@ let remove_Wl cclibs =
                  (String.sub cclib 4 (String.length cclib - 4))
     else cclib)
 
-let call_linker mode output_name files extra =
+let call_linker ?(native_toplevel = false) mode output_name files extra =
   Profile.record_call "c-linker" (fun () ->
     let cmd =
       if mode = Partial then
@@ -194,7 +194,8 @@ let call_linker mode output_name files extra =
           )
           (Filename.quote output_name)
           ""  (*(Clflags.std_include_flag "-I")*)
-          (quote_prefixed "-L" (Load_path.get_paths ()))
+          (if native_toplevel then ""
+           else quote_prefixed "-L" (Load_path.get_paths ()))
           (String.concat " " (List.rev !Clflags.all_ccopts))
           (quote_files files)
           extra

utils/ccomp.mli

@@ -35,6 +35,13 @@ type link_mode =
   | MainDll
   | Partial
 
-val call_linker: link_mode -> string -> string list -> string -> int
+(* If the ~native_toplevel flag is true, we don't pass any `-L` flags to gcc.
+   In some cases we observed so many flags being passed that gcc would crash,
+   but they should all be unnecessary as we're compiling with `-shared` in that
+   case. *)
+(* CR-someday ccasinghino: the argument above equally applies to all cases when
+   `link_mode` is `Dll`, but that didn't seem to work.  Understand why. *)
+val call_linker:
+  ?native_toplevel:bool -> link_mode -> string -> string list -> string -> int
 
 val linker_is_flexlink : bool