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

fusl/src/math/nexttoward.c

Index: fusl/src/math/nexttoward.c
diff --git a/fusl/src/math/nexttoward.c b/fusl/src/math/nexttoward.c
new file mode 100644
index 0000000000000000000000000000000000000000..827ee5c3c23ab0d9c33ff4205835bf068f0320e8
--- /dev/null
+++ b/fusl/src/math/nexttoward.c
@@ -0,0 +1,42 @@
+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+double nexttoward(double x, long double y)
+{
+	return nextafter(x, y);
+}
+#else
+double nexttoward(double x, long double y)
+{
+	union {double f; uint64_t i;} ux = {x};
+	int e;
+
+	if (isnan(x) || isnan(y))
+		return x + y;
+	if (x == y)
+		return y;
+	if (x == 0) {
+		ux.i = 1;
+		if (signbit(y))
+			ux.i |= 1ULL<<63;
+	} else if (x < y) {
+		if (signbit(x))
+			ux.i--;
+		else
+			ux.i++;
+	} else {
+		if (signbit(x))
+			ux.i++;
+		else
+			ux.i--;
+	}
+	e = ux.i>>52 & 0x7ff;
+	/* raise overflow if ux.f is infinite and x is finite */
+	if (e == 0x7ff)
+		FORCE_EVAL(x+x);
+	/* raise underflow if ux.f is subnormal or zero */
+	if (e == 0)
+		FORCE_EVAL(x*x + ux.f*ux.f);
+	return ux.f;
+}
+#endif