Add thunks for MojoGetRights(), etc.

I.e., this makes MojoGetRights() to apps.

R=azani@chromium.org

Review URL: https://codereview.chromium.org/1993283002 .
diff --git a/mojo/edk/embedder/system_impl_private_entrypoints.cc b/mojo/edk/embedder/system_impl_private_entrypoints.cc
index f35c91f..0f1b18d 100644
--- a/mojo/edk/embedder/system_impl_private_entrypoints.cc
+++ b/mojo/edk/embedder/system_impl_private_entrypoints.cc
@@ -92,6 +92,14 @@
   return core->Close(handle);
 }
 
+MojoResult MojoSystemImplGetRights(MojoSystemImpl system,
+                                   MojoHandle handle,
+                                   MojoHandleRights* rights) {
+  mojo::system::Core* core = static_cast<mojo::system::Core*>(system);
+  DCHECK(core);
+  return core->GetRights(handle, MakeUserPointer(rights));
+}
+
 MojoResult MojoSystemImplWait(MojoSystemImpl system,
                               MojoHandle handle,
                               MojoHandleSignals signals,