Add a "fork" of musl as //fusl.

fusl/src/thread/pthread_join.c

+#include "pthread_impl.h"
+#include <sys/mman.h>
+
+int __munmap(void *, size_t);
+void __pthread_testcancel(void);
+int __pthread_setcancelstate(int, int *);
+
+int __pthread_join(pthread_t t, void **res)
+{
+        int tmp, cs;
+        __pthread_testcancel();
+        __pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+        if (cs == PTHREAD_CANCEL_ENABLE) __pthread_setcancelstate(cs, 0);
+        while ((tmp = t->tid)) __timedwait_cp(&t->tid, tmp, 0, 0, 0);
+        __pthread_setcancelstate(cs, 0);
+        a_barrier();
+        if (res) *res = t->result;
+        if (t->map_base) __munmap(t->map_base, t->map_size);
+        return 0;
+}
+
+weak_alias(__pthread_join, pthread_join);