cxxrtl: store comb $print cell last EN/ARGS in module

statics were obviously wrong -- may be multiple instantiations of any
given module.  Extend test to cover this.

tests/fmt/always_comb_tb.v

@@ -1,7 +1,8 @@
 module tb;
     reg clk = 0;
 
-    top uut (.clk(clk));
+    top uut1 (.clk(clk));
+    top uut2 (.clk(clk));
 
     always #1 clk <= ~clk;
     initial #20 $finish;