Fix corlib self referencing bug.

Fix corlib self referencing bug.

In mcs/class/corlib/System.Reflection.Emit:

	* TypeBuilder.cs (DefineDefaultConstructor): If the resolved
	type is either typeof (object) or typeof (ValueType) we backout
	to the non resolved type as it might be one of the corlib's
	typebuilders.

In mcs/mcs:

	* rootcontext.cs (CloseTypes): Only apply the enum hack for
	corlib here.

	* rootcontext.cs (EmitCode): Do the corlib basic type hack
	here before types are emitted. This is required since EmitType
	produce type initializers, which derive from ValueType and must
	be able to see the TypeBuilder version of it.