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

fusl/src/stdio/__overflow.c

Index: fusl/src/stdio/__overflow.c
diff --git a/fusl/src/stdio/__overflow.c b/fusl/src/stdio/__overflow.c
new file mode 100644
index 0000000000000000000000000000000000000000..3bb37923abb847787878cf868d86bce570fb267e
--- /dev/null
+++ b/fusl/src/stdio/__overflow.c
@@ -0,0 +1,10 @@
+#include "stdio_impl.h"
+
+int __overflow(FILE *f, int _c)
+{
+	unsigned char c = _c;
+	if (!f->wend && __towrite(f)) return EOF;
+	if (f->wpos < f->wend && c != f->lbf) return *f->wpos++ = c;
+	if (f->write(f, &c, 1)!=1) return EOF;
+	return c;
+}