Merge pull request #3863 from lambdageek/dev-42584-take4

[custom-attrs] Add a low level iterator for custom attributes on assemblies

And use it to check for the reference assembly attribute.

Using the low level APIs means we don't trigger load of referenced
assemblies and we don't need to take locks, so it is safe to call this
during assembly loading.

Follow up work for https://bugzilla.xamarin.com/show_bug.cgi?id=42584