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

fusl/src/math/modff.c

+#include "libm.h"
+
+float modff(float x, float *iptr)
+{
+        union {float f; uint32_t i;} u = {x};
+        uint32_t mask;
+        int e = (int)(u.i>>23 & 0xff) - 0x7f;
+
+        /* no fractional part */
+        if (e >= 23) {
+                *iptr = x;
+                if (e == 0x80 && u.i<<9 != 0) { /* nan */
+                        return x;
+                }
+                u.i &= 0x80000000;
+                return u.f;
+        }
+        /* no integral part */
+        if (e < 0) {
+                u.i &= 0x80000000;
+                *iptr = u.f;
+                return x;
+        }
+
+        mask = 0x007fffff>>e;
+        if ((u.i & mask) == 0) {
+                *iptr = x;
+                u.i &= 0x80000000;
+                return u.f;
+        }
+        u.i &= ~mask;
+        *iptr = u.f;
+        return x - u.f;
+}