Get the normal event accessors generated (#89441)

Get the normal event accessors generated (#89441)

Because System.Threading.dll was not referenced, this led to Roslyn generating the Delegate.Combine based accessor which does not have any locals.  https://github.com/dotnet/roslyn/blob/520e2f10078ab0e5f85c78fdd95d786d78b9676a/src/Compilers/CSharp/Portable/Compiler/MethodBodySynthesizer.cs#L323-L520

In our UnityLinker test framework we have 2 cases currently.  Mono, which will reference mscorlib.dll, and Roslyn will find `System.Threading.Interlocked.CompareExchange` and use the normal approach.

And currently I have our coreclr tests referencing System.Threading.dll for no great reason.  This is how I stumbled on this.

While this change isn't strictly necessary, I think it's better to have coverage on the `System.Threading.Interlocked.CompareExchange` approach because that's what would normally be generated.  It's also the more complex case for the linker to handle since it has to clear the locals.