Upgrade csc ref to latest

Provides a few benefits:

1) The new package has the correct dependencies listed, so I removed the
-MissingDependenciesOK flag from the crossgen scripts.

2) The new compiler supports -publicSign (formerly called "OSS sign"),
which I patched through dotnet-compile-csc .

src/Microsoft.DotNet.Tools.Compiler.Csc/Program.cs

@@ -149,9 +149,10 @@ namespace Microsoft.DotNet.Tools.Compiler.Csc
                 commonArgs.Add("-delaysign");
             }
 
-            // TODO: What is this? What does it mean to sign without a key?
-            // Is this "OSS" signing?
-            // if (options.StrongName)
+            if (options.PublicSign == true)
+            {
+                commonArgs.Add("-publicsign");
+            }
 
             if (options.EmitEntryPoint != true)
             {