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

fusl/src/multibyte/c16rtomb.c

+#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;
+}