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

fusl/src/stdio/__towrite.c

+#include "stdio_impl.h"
+
+int __towrite(FILE *f)
+{
+        f->mode |= f->mode-1;
+        if (f->flags & (F_NOWR)) {
+                f->flags |= F_ERR;
+                return EOF;
+        }
+        /* Clear read buffer (easier than summoning nasal demons) */
+        f->rpos = f->rend = 0;
+
+        /* Activate write through the buffer. */
+        f->wpos = f->wbase = f->buf;
+        f->wend = f->buf + f->buf_size;
+
+        return 0;
+}
+
+void __stdio_exit_needed(void);
+
+void __towrite_needs_stdio_exit()
+{
+        __stdio_exit_needed();
+}