Add exit/_Exit to musl dynamic.list

Some DFX features need the ability to interpose exit and/or _Exit to
do some work at program exit.
For example, the appspawn service need to handle the exit processing
logic of all its children process to ensure no unexpected crash when
exiting.

Issue: #I6ZJJ3
Signed-off-by: NCaoruihong <crh.cao@huawei.com>

dynamic.list

@@ -44,4 +44,7 @@ __progname;
 __progname_full;
 
 __stack_chk_guard;
+
+exit;
+_Exit;
 };