ldbl-128ibm-compat: Provide nexttoward functions

The functions in the nexttoward family are special, in the sense that
they always have a long double argument, regardless of their suffix
(i.e.: nexttowardf and nexttoward have a long double argument, besides
the float and double arguments).

On top of that, they are also special because nexttoward functions are
not part of the _FloatN API, hence __nexttowardf128 do not exist.

This patch adds 4 new function implementations for the new long double
format:

  __nexttoward_to_ieee128
  __nexttowardf_to_ieee128
  __nexttowardieee128 (as an alias to __nextafterieee128)

Likewise, rename "long double" "_Float128" in shared ldbl-128
files to ensure correct type is used irrespective of ABI
switches.

Thank you to those who helped out with this patch:

Co-Authored-By: Tulio Magno Quites Machado Filho  <tuliom@linux.ibm.com>

sysdeps/ieee754/ldbl-128/s_nexttoward.c

@@ -31,7 +31,7 @@ static char rcsid[] = "$NetBSD: $";
 #include <math_private.h>
 #include <float.h>
 
-double __nexttoward(double x, long double y)
+double __nexttoward(double x, _Float128 y)
 {
 	int32_t hx,ix;
 	int64_t hy,iy;
@@ -47,7 +47,7 @@ double __nexttoward(double x, long double y)
 	   ((iy>=0x7fff000000000000LL)&&((iy-0x7fff000000000000LL)|ly)!=0))
 							    /* y is nan */
 	   return x+y;
-	if((long double) x==y) return y;	/* x=y, return y */
+	if((_Float128) x==y) return y;		/* x=y, return y */
 	if((ix|lx)==0) {			/* x == 0 */
 	    double u;
 	    INSERT_WORDS(x,(uint32_t)((hy>>32)&0x80000000),1);/* return +-minsub */

sysdeps/ieee754/ldbl-128/s_nexttowardf.c

@@ -23,7 +23,7 @@ static char rcsid[] = "$NetBSD: $";
 #include <math-barriers.h>
 #include <math_private.h>
 
-float __nexttowardf(float x, long double y)
+float __nexttowardf(float x, _Float128 y)
 {
 	int32_t hx,ix;
 	int64_t hy,iy;
@@ -38,7 +38,7 @@ float __nexttowardf(float x, long double y)
 	   ((iy>=0x7fff000000000000LL)&&((iy-0x7fff000000000000LL)|ly)!=0))
 				/* y is nan */
 	   return x+y;
-	if((long double) x==y) return y;	/* x=y, return y */
+	if((_Float128) x==y) return y;		/* x=y, return y */
 	if(ix==0) {				/* x == 0 */
 	    float u;
 	    SET_FLOAT_WORD(x,(uint32_t)((hy>>32)&0x80000000)|1);/* return +-minsub*/

sysdeps/ieee754/ldbl-128ibm-compat/Versions

@@ -91,6 +91,9 @@ libm {
     __nearbyintieee128;
     __nextafterieee128;
     __nextdownieee128;
+    __nexttowardf_to_ieee128;
+    __nexttowardieee128;
+    __nexttoward_to_ieee128;
     __nextupieee128;
     __powieee128;
     __remainderieee128;

sysdeps/ieee754/ldbl-128ibm-compat/s_nextafterf128.c

@@ -0,0 +1,20 @@
+/* Provide nexttowardl and nextafterl implementations for IEEE long double.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+#include "../float128/s_nextafterf128.c"
+
+strong_alias (__nextafterieee128, __nexttowardieee128)

sysdeps/ieee754/ldbl-128ibm-compat/s_nexttowardf128.c

@@ -0,0 +1,38 @@
+/* Provide nexttoward[|f] implementations for IEEE long double.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <float128_private.h>
+
+/* Build nexttoward functions with binary128 format.  */
+#undef weak_alias
+#define weak_alias(from, to)
+#undef libm_alias_ldouble
+#define libm_alias_ldouble(from, to)
+#undef __nexttoward
+#define __nexttoward __nexttoward_to_ieee128
+#include <sysdeps/ieee754/ldbl-128/s_nexttoward.c>
+
+#undef weak_alias
+#define weak_alias(from, to)
+#undef libm_alias_ldouble
+#define libm_alias_ldouble(from, to)
+#undef __nexttowardf
+#define __nexttowardf __nexttowardf_to_ieee128
+#include <sysdeps/ieee754/ldbl-128/s_nexttowardf.c>
+
+#include <libm-alias-ldouble.h>