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

fusl/src/multibyte/c16rtomb.c

Index: fusl/src/multibyte/c16rtomb.c
diff --git a/fusl/src/multibyte/c16rtomb.c b/fusl/src/multibyte/c16rtomb.c
new file mode 100644
index 0000000000000000000000000000000000000000..39ca3758fa16eb16d12f468046045f0f425e5d1c
--- /dev/null
+++ b/fusl/src/multibyte/c16rtomb.c
@@ -0,0 +1,35 @@
+#include <uchar.h>
+#include <errno.h>
+#include <wchar.h>
+
+size_t c16rtomb(char *restrict s, char16_t c16, mbstate_t *restrict ps)
+{
+	static unsigned internal_state;
+	if (!ps) ps = (void *)&internal_state;
+	unsigned *x = (unsigned *)ps;
+	wchar_t wc;
+
+	if (!s) {
+		if (*x) goto ilseq;
+		return 1;
+	}
+
+	if (!*x && c16 - 0xd800u < 0x400) {
+		*x = c16 - 0xd7c0 << 10;
+		return 0;
+	}
+
+	if (*x) {
+		if (c16 - 0xdc00u >= 0x400) goto ilseq;
+		else wc = *x + c16 - 0xdc00;
+		*x = 0;
+	} else {
+		wc = c16;
+	}
+	return wcrtomb(s, wc, 0);
+
+ilseq:
+	*x = 0;
+	errno = EILSEQ;
+	return -1;
+}