declare and make hidden additional internal stdio symbols

src/internal/stdio_impl.h

@@ -50,6 +50,10 @@ struct _IO_FILE {
 	struct __locale_struct *locale;
 };
 
+extern hidden FILE *volatile __stdin_used;
+extern hidden FILE *volatile __stdout_used;
+extern hidden FILE *volatile __stderr_used;
+
 hidden size_t __stdio_read(FILE *, unsigned char *, size_t);
 hidden size_t __stdio_write(FILE *, const unsigned char *, size_t);
 hidden size_t __stdout_write(FILE *, const unsigned char *, size_t);
@@ -61,6 +65,7 @@ hidden size_t __string_read(FILE *, unsigned char *, size_t);
 hidden int __toread(FILE *);
 hidden int __towrite(FILE *);
 
+hidden void __stdio_exit(void);
 hidden void __stdio_exit_needed(void);
 
 #if defined(__PIC__) && (100*__GNUC__+__GNUC_MINOR__ >= 303)