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

fusl/src/multibyte/mbrtoc16.c

+#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;
+}