Add unstable -Zdefault-hidden-visibility cmdline flag for rustc.

The new flag has been described in the Major Change Proposal at
https://github.com/rust-lang/compiler-team/issues/656

compiler/rustc_monomorphize/src/partitioning.rs

@@ -883,7 +883,7 @@ fn mono_item_visibility<'tcx>(
 }
 
 fn default_visibility(tcx: TyCtxt<'_>, id: DefId, is_generic: bool) -> Visibility {
-    if !tcx.sess.target.default_hidden_visibility {
+    if !tcx.sess.default_hidden_visibility() {
         return Visibility::Default;
     }