Cycles:

* Revert r57203 (len() renaming)
There seems to be a problem with nVidia OpenCL after this and I haven't figured out the real cause yet. 
Better to selectively enable native length() later, after figuring out what's wrong. 

This fixes [#35612].

intern/cycles/render/mesh.cpp

@@ -187,7 +187,7 @@ void Mesh::add_face_normals()
 			float3 v2 = verts_ptr[t.v[2]];
 
 			float3 norm = cross(v1 - v0, v2 - v0);
-			float normlen = length(norm);
+			float normlen = len(norm);
 			if(normlen == 0.0f)
 				fN[i] = make_float3(0.0f, 0.0f, 0.0f);
 			else