Add a new helper function to record varargs info when va_end is called.

This will be used in conjunction with the new implementation of variable arguments, where they are not removed from the stack until the end of the function.

stdlib.asm

@@ -1651,8 +1651,37 @@ empty    ds    2
 
 ****************************************************************
 *
-*  void __va_end(list)
-*        va_list list;
+*  void __record_va_info(va_list ap);
+*
+*  Record that a traversal of variable arguments has finished.
+*  Data is recorded in the internal va info that will be used
+*  to remove variable arguments at the end of the function.
+*
+*  Inputs:
+*        ap - the va_list
+*
+****************************************************************
+*
+__record_va_info start
+va_info_ptr equ 1                       pointer to the internal va info
+
+         csubroutine (4:ap),4
+         ldy   #4                       get pointer to internal va info
+         lda   [ap],y
+         sta   va_info_ptr
+         stz   va_info_ptr+2
+
+         lda   [ap]                     update end of variable arguments
+         cmp   [va_info_ptr]
+         blt   ret
+         sta   [va_info_ptr]
+
+ret      creturn
+         end
+
+****************************************************************
+*
+*  void __va_end(internal_va_info *list);
 *
 *  Remove variable length arguments from the stack.
 *