The constructor of AssemblyLoadContext was missing a call to
GC.SuppressFinalize for non-collectible AssemblyLoadContext. That means
that the finalizer can be executed and unloading being initialized even
for non-collectible AssemblyLoadContext.
This change fixes that.

The finalizer for the non-collectible AssemblyLoadContext is never
called and so it should not be in the finalization queue.

Check if the constructor was executed before we run the finalizer code.