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

fusl/src/math/truncl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double truncl(long double x)
+{
+        return trunc(x);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+
+static const long double toint = 1/LDBL_EPSILON;
+
+long double truncl(long double x)
+{
+        union ldshape u = {x};
+        int e = u.i.se & 0x7fff;
+        int s = u.i.se >> 15;
+        long double y;
+
+        if (e >= 0x3fff+LDBL_MANT_DIG-1)
+                return x;
+        if (e <= 0x3fff-1) {
+                FORCE_EVAL(x + 0x1p120f);
+                return x*0;
+        }
+        /* y = int(|x|) - |x|, where int(|x|) is an integer neighbor of |x| */
+        if (s)
+                x = -x;
+        y = x + toint - toint - x;
+        if (y > 0)
+                y -= 1;
+        x += y;
+        return s ? -x : x;
+}
+#endif