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

fusl/src/math/llrintl.c

+#include <limits.h>
+#include <fenv.h>
+#include "libm.h"
+
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long long llrintl(long double x)
+{
+        return llrint(x);
+}
+#elif defined(FE_INEXACT)
+/*
+see comments in lrint.c
+
+Note that if LLONG_MAX == 0x7fffffffffffffff && LDBL_MANT_DIG == 64
+then x == 2**63 - 0.5 is the only input that overflows and
+raises inexact (with tonearest or upward rounding mode)
+*/
+long long llrintl(long double x)
+{
+        #pragma STDC FENV_ACCESS ON
+        int e;
+
+        e = fetestexcept(FE_INEXACT);
+        x = rintl(x);
+        if (!e && (x > LLONG_MAX || x < LLONG_MIN))
+                feclearexcept(FE_INEXACT);
+        /* conversion */
+        return x;
+}
+#else
+long long llrintl(long double x)
+{
+        return rintl(x);
+}
+#endif