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

fusl/src/stdlib/fcvt.c

+#define _GNU_SOURCE
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+char *fcvt(double x, int n, int *dp, int *sign)
+{
+        char tmp[1500];
+        int i, lz;
+
+        if (n > 1400U) n = 1400;
+        sprintf(tmp, "%.*f", n, x);
+        i = (tmp[0] == '-');
+        if (tmp[i] == '0') lz = strspn(tmp+i+2, "0");
+        else lz = -(int)strcspn(tmp+i, ".");
+
+        if (n<=lz) {
+                *sign = i;
+                *dp = 1;
+                if (n>14U) n = 14;
+                return "000000000000000"+14-n;
+        }
+
+        return ecvt(x, n-lz, dp, sign);
+}