add cxxrtl platform

src/groovylight/platforms/cxxrtl_sim.py

@@ -0,0 +1,9 @@
+# CXXRTL-based simulator.
+# Uses SDL2 + black-box network stack to simulate the entire design.
+
+
+# Tasks:
+# Generate header with network port information
+# black box the UDP streaming.
+# provide code for display outputs to render onto SDL2 canvas.
+# compile code (optionally)