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

fusl/src/math/acoshl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double acoshl(long double x)
+{
+        return acosh(x);
+}
+#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
+/* acosh(x) = log(x + sqrt(x*x-1)) */
+long double acoshl(long double x)
+{
+        union ldshape u = {x};
+        int e = u.i.se & 0x7fff;
+
+        if (e < 0x3fff + 1)
+                /* |x| < 2, invalid if x < 1 or nan */
+                return log1pl(x-1 + sqrtl((x-1)*(x-1)+2*(x-1)));
+        if (e < 0x3fff + 32)
+                /* |x| < 0x1p32 */
+                return logl(2*x - 1/(x+sqrtl(x*x-1)));
+        return logl(x) + 0.693147180559945309417232121458176568L;
+}
+#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
+// TODO: broken implementation to make things compile
+long double acoshl(long double x)
+{
+        return acosh(x);
+}
+#endif