S390: Fix build failure in test string/tst-endian.c with gcc 6.

Building string/tst-endian.c with gcc 6 produces an build warning/error on s390 (big endian machine):
gcc tst-endian.c -c -std=gnu11 -fgnu89-inline  -O2 or -O3 ...
tst-endian.c: In function ‘do_test’:
tst-endian.c:16:30: error: self-comparison always evaluates to false [-Werror=tautological-compare]
    if (htobe16 (be16toh (i)) != i)
                              ^~
...

See definitions of htobexx, bexxtoh in string/endian.h:
...

This patch silences these warnings with DIAG_* macros if build with gcc 6
and newer.

The same warnings occur on little endian machines with the
"htoleXX (leXXtoh (i)) != i" if-statements.

ChangeLog:

	* string/tst-endian.c: Include <libc-internal.h>.
	(do_test): Ignore tautological-compare warnings around
	"htobeXX (beXXtoh (i)) != i" and
	"htoleXX (leXXtoh (i)) != i" if-statements.

1 files changed, 7 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 6c21184d88..9bdcb5146c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2016-01-22  Stefan Liebler  <stli@linux.vnet.ibm.com>
+
+	* string/tst-endian.c: Include <libc-internal.h>.
+	(do_test): Ignore tautological-compare warnings around
+	"htobeXX (beXXtoh (i)) != i" and
+	"htoleXX (leXXtoh (i)) != i" if-statements.
+
 2016-01-24  David S. Miller  <davem@davemloft.net>
 
 	* sysdeps/sparc/sparc32/fpu/e_sqrtl.c: New file.