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

fusl/src/multibyte/mbrtoc32.c

Index: fusl/src/multibyte/mbrtoc32.c
diff --git a/fusl/src/multibyte/mbrtoc32.c b/fusl/src/multibyte/mbrtoc32.c
new file mode 100644
index 0000000000000000000000000000000000000000..9b6b2367398e426815dc8f0390daae3c6957779d
--- /dev/null
+++ b/fusl/src/multibyte/mbrtoc32.c
@@ -0,0 +1,13 @@
+#include <uchar.h>
+#include <wchar.h>
+
+size_t mbrtoc32(char32_t *restrict pc32, const char *restrict s, size_t n, mbstate_t *restrict ps)
+{
+	static unsigned internal_state;
+	if (!ps) ps = (void *)&internal_state;
+	if (!s) return mbrtoc32(0, "", 1, ps);
+	wchar_t wc;
+	size_t ret = mbrtowc(&wc, s, n, ps);
+	if (ret <= 4 && pc32) *pc32 = wc;
+	return ret;
+}