Add a C++ backend for LibertyXDR

Also change the C backend so that it also de/serializes
unions without any other fields besides the tag.

tools/lxdrgen-go.awk

@@ -439,7 +439,7 @@ function codegen_struct(name, cg,    gotype) {
 		delete cg[i]
 }
 
-function codegen_union_tag(d, cg) {
+function codegen_union_tag(name, d, cg) {
 	cg["tagtype"] = d["type"]
 	cg["tagname"] = snaketocamel(d["name"])
 	# The tag is implied from the type of struct stored in the interface.