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

fusl/src/math/copysignl.c

Index: fusl/src/math/copysignl.c
diff --git a/fusl/src/math/copysignl.c b/fusl/src/math/copysignl.c
new file mode 100644
index 0000000000000000000000000000000000000000..9dd933cfa9c55214df9392e9a90dcc83a2dd0c8f
--- /dev/null
+++ b/fusl/src/math/copysignl.c
@@ -0,0 +1,16 @@
+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double copysignl(long double x, long double y)
+{
+	return copysign(x, y);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+long double copysignl(long double x, long double y)
+{
+	union ldshape ux = {x}, uy = {y};
+	ux.i.se &= 0x7fff;
+	ux.i.se |= uy.i.se & 0x8000;
+	return ux.f;
+}
+#endif