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

fusl/src/math/modf.c

+#include "libm.h"
+
+double modf(double x, double *iptr)
+{
+        union {double f; uint64_t i;} u = {x};
+        uint64_t mask;
+        int e = (int)(u.i>>52 & 0x7ff) - 0x3ff;
+
+        /* no fractional part */
+        if (e >= 52) {
+                *iptr = x;
+                if (e == 0x400 && u.i<<12 != 0) /* nan */
+                        return x;
+                u.i &= 1ULL<<63;
+                return u.f;
+        }
+
+        /* no integral part*/
+        if (e < 0) {
+                u.i &= 1ULL<<63;
+                *iptr = u.f;
+                return x;
+        }
+
+        mask = -1ULL>>12>>e;
+        if ((u.i & mask) == 0) {
+                *iptr = x;
+                u.i &= 1ULL<<63;
+                return u.f;
+        }
+        u.i &= ~mask;
+        *iptr = u.f;
+        return x - u.f;
+}