libthr: Create landing pad in thr_once in C++ exception

In _thr_once, if init_routine fails with exception, the stack allocate
__cup will become invalid when unwinder walk back and free the stack of
thr_once. This cause cleanup linked list be dangling. In here, we give
the cleanup function a landing pad so that we td can correctly point
to previous cleanup function.

Reviewed by:    kib
MFC after:      2 weeks
Differential Revision: https://reviews.freebsd.org/D61212

lib/libthr/thread/thr_private.h

@@ -249,7 +249,9 @@ struct pthread_cleanup {
 };
 
 #define	THR_CLEANUP_PUSH(td, func, arg) {		\
-	struct pthread_cleanup __cup;			\
+	struct pthread_cleanup __cup			\
+	    __attribute__((__cleanup__(			\
+	    _thr_cleanup_unwind)));			\
 							\
 	__cup.routine = func;				\
 	__cup.routine_arg = arg;			\
@@ -949,6 +951,18 @@ _thr_check_init(void)
 		_libpthread_init(NULL);
 }
 
+static inline void
+_thr_cleanup_unwind(struct pthread_cleanup *cup)
+{
+	struct pthread *curthread;
+
+	curthread = _get_curthread();
+	if (curthread->cleanup == cup) {
+		curthread->cleanup = cup->prev;
+		cup->routine(cup->routine_arg);
+	}
+}
+
 struct wake_addr *_thr_alloc_wake_addr(void);
 void	_thr_release_wake_addr(struct wake_addr *);
 int	_thr_sleep(struct pthread *, int, const struct timespec *);