Add basic Hello World program for easier testing

We can use this to assert simple things about .NET Core (our build in
particular) for easier debugging.

scripts/HelloWorld.cs

@@ -0,0 +1,9 @@
+using System;
+
+internal class Program
+{
+    private static void Main()
+    {
+        Console.WriteLine("Hello World!");
+    }
+}

scripts/Makefile

@@ -185,7 +185,7 @@ run-file: $(RUN_TARGETS) internal-prepare-exec_env internal-prepare-clr
 # compiles "Hello World" like executables using .NET Core
 %.exe: %.cs prepare internal-prepare-clr
 	$(CSC) -out:$@ $(CSCOPTS_BASE) -target:exe $(COREREF) $<
-	@echo run with monad-tty ./scripts/exec_env/coreclr/corerun ./scripts/$@
+	@echo run with monad-tty ./exec_env/coreclr/corerun ./$@
 
 run-debugclr: $(RUN_TARGETS) internal-prepare-exec_env internal-prepare-clr
 	PAL_DBG_CHANNELS="+LOADER.TRACE" $(APP_BASE)/runps-simple.sh get-location