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

fusl/src/math/floorl.c

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