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

fusl/src/thread/thrd_join.c

Index: fusl/src/thread/thrd_join.c
diff --git a/fusl/src/thread/thrd_join.c b/fusl/src/thread/thrd_join.c
new file mode 100644
index 0000000000000000000000000000000000000000..ac6678939f6b3436818936eaf56b395479c4f3c2
--- /dev/null
+++ b/fusl/src/thread/thrd_join.c
@@ -0,0 +1,12 @@
+#include <stdint.h>
+#include <threads.h>
+
+int __pthread_join(thrd_t, void**);
+
+int thrd_join(thrd_t t, int *res)
+{
+        void *pthread_res;
+        __pthread_join(t, &pthread_res);
+        if (res) *res = (int)(intptr_t)pthread_res;
+        return thrd_success;
+}