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.cc

@@ -3,11 +3,14 @@
 
 int main()
 {
-	cxxrtl_design::p_top uut;
+	cxxrtl_design::p_top uut1, uut2;
 
 	for (int i = 0; i < 20; ++i) {
-		uut.p_clk.set(!uut.p_clk);
-		uut.step();
+		uut1.p_clk.set(!uut1.p_clk);
+		uut1.step();
+
+		uut2.p_clk.set(!uut2.p_clk);
+		uut2.step();
 	}
 
 	return 0;

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;