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

fusl/src/multibyte/mbrtoc16.c

Index: fusl/src/multibyte/mbrtoc16.c
diff --git a/fusl/src/multibyte/mbrtoc16.c b/fusl/src/multibyte/mbrtoc16.c
new file mode 100644
index 0000000000000000000000000000000000000000..765ff9037cd3554db14e275650adb7c807632777
--- /dev/null
+++ b/fusl/src/multibyte/mbrtoc16.c
@@ -0,0 +1,30 @@
+#include <uchar.h>
+#include <wchar.h>
+
+size_t mbrtoc16(char16_t *restrict pc16, const char *restrict s, size_t n, mbstate_t *restrict ps)
+{
+	static unsigned internal_state;
+	if (!ps) ps = (void *)&internal_state;
+	unsigned *pending = (unsigned *)ps;
+
+	if (!s) return mbrtoc16(0, "", 1, ps);
+
+	/* mbrtowc states for partial UTF-8 characters have the high bit set;
+	 * we use nonzero states without high bit for pending surrogates. */
+	if ((int)*pending > 0) {
+ 		if (pc16) *pc16 = *pending;
+		*pending = 0;
+		return -3;
+	}
+
+	wchar_t wc;
+	size_t ret = mbrtowc(&wc, s, n, ps);
+	if (ret <= 4) {
+		if (wc >= 0x10000) {
+			*pending = (wc & 0x3ff) + 0xdc00;
+			wc = 0xd7c0 + (wc >> 10);
+		}
+		if (pc16) *pc16 = wc;
+	}
+	return ret;
+}