S390: Fix build error with gcc6 in utf8_utf16-z9.c.

This patch fixes the build error with gcc6:
array subscript is above array bounds [-Werror=array-bounds]

While including loop.c to construct the SINGLE(LOOPFCT) method
for converting from UTF-16 to UTF-8, the bytebuf array with length
MAX_NEEDED_INPUT is used as inptr. MAX_NEEDED_INPUT defaults to
MIN_NEEDED_INPUT if not defined before including loop.c.
Thus bytebuf has a length of 2.
This patch defines MAX_NEEDED_INPUT to MAX_NEEDED_TO, which is 4.

ChangeLog:

	* sysdeps/s390/s390-64/utf8-utf16-z9.c
	(MAX_NEEDED_INPUT): New define.
	(MAX_NEEDED_OUTPUT): New define.

ChangeLog

@@ -1,3 +1,9 @@
+2015-08-26  Stefan Liebler  <stli@linux.vnet.ibm.com>
+
+	* sysdeps/s390/s390-64/utf8-utf16-z9.c
+	(MAX_NEEDED_INPUT): New define.
+	(MAX_NEEDED_OUTPUT): New define.
+
 2015-08-26  Stefan Liebler  <stli@linux.vnet.ibm.com>
 
 	* NEWS: New item for IBM z13 string optimizations.

sysdeps/s390/s390-64/utf8-utf16-z9.c

@@ -183,6 +183,7 @@ gconv_end (struct __gconv_step *data)
 #define MIN_NEEDED_INPUT	MIN_NEEDED_FROM
 #define MAX_NEEDED_INPUT	MAX_NEEDED_FROM
 #define MIN_NEEDED_OUTPUT	MIN_NEEDED_TO
+#define MAX_NEEDED_OUTPUT	MAX_NEEDED_TO
 #define LOOPFCT			FROM_LOOP
 /* The software implementation is based on the code in gconv_simple.c.  */
 #define BODY								\
@@ -340,6 +341,7 @@ gconv_end (struct __gconv_step *data)
 /* Conversion from UTF-16 to UTF-8.  */
 
 #define MIN_NEEDED_INPUT	MIN_NEEDED_TO
+#define MAX_NEEDED_INPUT	MAX_NEEDED_TO
 #define MIN_NEEDED_OUTPUT	MIN_NEEDED_FROM
 #define MAX_NEEDED_OUTPUT	MAX_NEEDED_FROM
 #define LOOPFCT			TO_LOOP