Don't shutdown in mono_runtime_try_shutdown

The Mono implementation of mono_runtime_try_shutdown does not actually
shut down the VM. Instead, it sets the flags to indicate the VM is
shutting down. The VM should actually shutdown in mono_runtime_quit.

In some cases, we see this code path taken when processing the
CMD_VM_EXIT command. This seems unlikely, but can happen when the code
above cannot find a suspended managed thread. If
mono_runtime_try_shutdown actually waits for managed threads to
complete, it will hang, as the main thread won't be suspended because
resume_vm is called just above mono_runtime_try_shutdown.

So for IL2CPP mono_runtime_try_shutdown doesn't do anything, but
mono_runtime_quit will actually shutdown the VM.

Author: Josh Peterson

mono/mini/il2cpp-stubs.cpp

@@ -361,6 +361,7 @@ void il2cpp_mono_runtime_quit (void)
 {
 #if UNITY_TINY
     IL2CPP_ASSERT(0 && "Not implemented yet for tiny");
+    return 0;
 #else
     il2cpp::vm::Runtime::Shutdown();
 #endif