cgo: implement C.struct_ types

These types (called elaborated types in C) are used as part of linked
lists, among others.

This is part an extra feature (to be compatible with CGo C.struct_
types) and part a bugfix: linked lists would result in endless recursion
leading to a stack overflow.

testdata/cgo/main.h

@@ -12,6 +12,12 @@ typedef struct collection {
 	unsigned char c;
 } collection_t;
 
+// linked list
+typedef struct list_t {
+	int           n;
+	struct list_t *next;
+} list_t;
+
 typedef union joined {
 	myint s;
 	float f;