fix namespace issue in pthread_join affecting thrd_join

pthread_testcancel is not in the ISO C reserved namespace and thus
cannot be used here. use the namespace-protected version of the
function instead.

src/thread/pthread_join.c

@@ -2,11 +2,12 @@
 #include <sys/mman.h>
 
 int __munmap(void *, size_t);
+void __pthread_testcancel(void);
 
 int __pthread_join(pthread_t t, void **res)
 {
 	int tmp;
-	pthread_testcancel();
+	__pthread_testcancel();
 	while ((tmp = t->tid)) __timedwait_cp(&t->tid, tmp, 0, 0, 0);
 	if (res) *res = t->result;
 	if (t->map_base) __munmap(t->map_base, t->map_size);