[CRT:MATH] Import nextafter, nextafterf and nexttowardf from musl

Note: On Windows, where a long double is the same as a double, nextafterl is the same as nextafter, nexttowardl is the same as nexttoward.
Also nexttoward is the same as nextafter.

sdk/lib/crt/float/float.cmake

@@ -6,7 +6,6 @@ list(APPEND CRT_FLOAT_SOURCE
     float/copysign.c
     float/fpclass.c
     float/fpecode.c
-    float/nafter.c
     float/scalb.c
 )
 

sdk/lib/crt/float/nafter.c

@@ -1,26 +0,0 @@
-/*
- * COPYRIGHT:   See COPYING in the top level directory
- * PROJECT:     ReactOS system libraries
- * FILE:        lib/sdk/crt/float/nafter.c
- * PURPOSE:     Unknown
- * PROGRAMER:   Unknown
- * UPDATE HISTORY:
- *              25/11/05: Added license header
- */
-
-#include <precomp.h>
-
-/*
- * @implemented
- */
-double _nextafter( double x, double y )
-{
-    WARN("This function is not implemented correctly\n");
-	if ( x == y)
-		return x;
-
-	if ( _isnan(x) || _isnan(y) )
-		return x;
-
-	return x;
-}