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

fusl/src/math/cosl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double cosl(long double x) {
+        return cos(x);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+long double cosl(long double x)
+{
+        union ldshape u = {x};
+        unsigned n;
+        long double y[2], hi, lo;
+
+        u.i.se &= 0x7fff;
+        if (u.i.se == 0x7fff)
+                return x - x;
+        x = u.f;
+        if (x < M_PI_4) {
+                if (u.i.se < 0x3fff - LDBL_MANT_DIG)
+                        /* raise inexact if x!=0 */
+                        return 1.0 + x;
+                return __cosl(x, 0);
+        }
+        n = __rem_pio2l(x, y);
+        hi = y[0];
+        lo = y[1];
+        switch (n & 3) {
+        case 0:
+                return __cosl(hi, lo);
+        case 1:
+                return -__sinl(hi, lo, 1);
+        case 2:
+                return -__cosl(hi, lo);
+        case 3:
+        default:
+                return __sinl(hi, lo, 1);
+        }
+}
+#endif