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

fusl/src/math/hypotl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double hypotl(long double x, long double y)
+{
+        return hypot(x, y);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+#if LDBL_MANT_DIG == 64
+#define SPLIT (0x1p32L+1)
+#elif LDBL_MANT_DIG == 113
+#define SPLIT (0x1p57L+1)
+#endif
+
+static void sq(long double *hi, long double *lo, long double x)
+{
+        long double xh, xl, xc;
+        xc = x*SPLIT;
+        xh = x - xc + xc;
+        xl = x - xh;
+        *hi = x*x;
+        *lo = xh*xh - *hi + 2*xh*xl + xl*xl;
+}
+
+long double hypotl(long double x, long double y)
+{
+        union ldshape ux = {x}, uy = {y};
+        int ex, ey;
+        long double hx, lx, hy, ly, z;
+
+        ux.i.se &= 0x7fff;
+        uy.i.se &= 0x7fff;
+        if (ux.i.se < uy.i.se) {
+                ex = uy.i.se;
+                ey = ux.i.se;
+                x = uy.f;
+                y = ux.f;
+        } else {
+                ex = ux.i.se;
+                ey = uy.i.se;
+                x = ux.f;
+                y = uy.f;
+        }
+
+        if (ex == 0x7fff && isinf(y))
+                return y;
+        if (ex == 0x7fff || y == 0)
+                return x;
+        if (ex - ey > LDBL_MANT_DIG)
+                return x + y;
+
+        z = 1;
+        if (ex > 0x3fff+8000) {
+                z = 0x1p10000L;
+                x *= 0x1p-10000L;
+                y *= 0x1p-10000L;
+        } else if (ey < 0x3fff-8000) {
+                z = 0x1p-10000L;
+                x *= 0x1p10000L;
+                y *= 0x1p10000L;
+        }
+        sq(&hx, &lx, x);
+        sq(&hy, &ly, y);
+        return z*sqrtl(ly+lx+hy+hx);
+}
+#endif