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

fusl/src/dirent/readdir_r.c

+#include <dirent.h>
+#include <errno.h>
+#include <string.h>
+#include "__dirent.h"
+#include "libc.h"
+
+int readdir_r(DIR *restrict dir, struct dirent *restrict buf, struct dirent **restrict result)
+{
+        struct dirent *de;
+        int errno_save = errno;
+        int ret;
+        
+        LOCK(dir->lock);
+        errno = 0;
+        de = readdir(dir);
+        if ((ret = errno)) {
+                UNLOCK(dir->lock);
+                return ret;
+        }
+        errno = errno_save;
+        if (de) memcpy(buf, de, de->d_reclen);
+        else buf = NULL;
+
+        UNLOCK(dir->lock);
+        *result = buf;
+        return 0;
+}
+
+LFS64_2(readdir_r, readdir64_r);