@nameof compiles incorrectly in mcs

## Steps to Reproduce

Test case:

```csharp
class M {
	static void Main (string[] args) {
		System.Console.WriteLine (@nameof (args));
	}
}
```

Compiles fine with mcs:

```shell
$ mcs test.cs
```

but not so much with csc:

```shell
$ csc test.cs
test.cs(3,29): error CS0103: The name 'nameof' does not exist in the current context
```
