Add whitebox example

src/main/scala/vexriscv/demo/WhiteboxPlugin.scala

@@ -0,0 +1,14 @@
+package vexriscv.demo
+
+import spinal.core._
+import vexriscv.plugin.Plugin
+import vexriscv.{DecoderService, Stageable, VexRiscv}
+
+class WhiteboxPlugin extends Plugin[VexRiscv]{
+  override def build(pipeline: VexRiscv): Unit = {
+    Component.current.afterElaboration {
+      def export(name : String): Unit = out(Component.current.reflectBaseType(name))
+      export("IBusCachedPlugin_fetchPc_pc")
+    }
+  }
+}