Fix zscale testing

Use the following command in vsim:

make CONFIG=ZscaleConfig MODEL=ZscaleTop TB=ZscaleTestHarness run-asm-tests

csrc/vcs_main.ZscaleTestHarness.cc

@@ -0,0 +1,12 @@
+// See LICENSE for license details.
+
+extern "C" {
+
+extern int vcs_main(int argc, char** argv);
+
+int main(int argc, char** argv)
+{
+  return vcs_main(argc, argv);
+}
+
+}