Consistently Use ui_file parameter to show callbacks

I happened to notice that one "show" callback was printing to
gdb_stdout rather than to the passed-in ui_file parameter.  I went
through all such callbacks and fixed them to consistently use the
ui_file.

Regression tested on x86-64 Fedora 34.

gdb/exec.c

@@ -102,7 +102,7 @@ static void
 show_exec_file_mismatch_command (struct ui_file *file, int from_tty,
 				 struct cmd_list_element *c, const char *value)
 {
-  fprintf_filtered (gdb_stdout,
+  fprintf_filtered (file,
 		    _("exec-file-mismatch handling is currently \"%s\".\n"),
 		    exec_file_mismatch_names[exec_file_mismatch_mode]);
 }