[runtime] Check for ReferenceAssemblyAttribute when loading for execution.

[runtime] Check for ReferenceAssemblyAttribute when loading for execution.

An assembly with a System.Runtime.CompilerServices.ReferenceAssemblyAttribute
may be loaded in a reflection-only context, but not for execution.

When loading assembly refs, or when loading assemblies by name (as
opposed to file path), a reference assembly will be skipped over and
other assemblies from the GAC and the load path will be tried.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=42584