Fix ldbl-128ibm nextafterl, nexttowardl sign of zero result (bug 19678).

The ldbl-128ibm implementation of nextafterl / nexttowardl returns -0
in FE_DOWNWARD mode when taking the next value below the least
positive subnormal, when it should return +0.  This patch fixes it to
check explicitly for this case.

Tested for powerpc.

	[BZ #19678]
	* sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c (__nextafterl):
	Ensure +0.0 is returned when taking the next value below the least
	positive value.

ChangeLog

@@ -1,3 +1,10 @@
+2016-02-19  Joseph Myers  <joseph@codesourcery.com>
+
+	[BZ #19678]
+	* sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c (__nextafterl):
+	Ensure +0.0 is returned when taking the next value below the least
+	positive value.
+
 2016-02-19  Florian Weimer  <fweimer@redhat.com>
 
 	* sysdeps/generic/malloc-machine.h: Assume mutex_init is always

sysdeps/ieee754/ldbl-128ibm/s_nextafterl.c

@@ -87,6 +87,9 @@ long double __nextafterl(long double x, long double y)
 		math_force_eval (u);		/* raise underflow flag */
 		__set_errno (ERANGE);
 	      }
+	      /* Avoid returning -0 in FE_DOWNWARD mode.  */
+	      if (x == 0.0L)
+		return 0.0L;
 	      return x;
 	    }
 	    /* If the high double is an exact power of two and the low