Fix string/tst-xbzero-opt if build with gcc head.

On s390x, the test string/tst-xbzero-opt is failing if build with gcc head:
FAIL: no clear/prepare: expected 32 got 0
FAIL: no clear/test: expected some got 0
FAIL: ordinary clear/prepare: expected 32 got 0
INFO: ordinary clear/test: found 0 patterns (memset not eliminated)
PASS: explicit clear/prepare: expected 32 got 32
PASS: explicit clear/test: expected 0 got 0

In setup_no_clear / setup_ordinary_clear, GCC is omitting the memcpy loop
in prepare_test_buffer. Thus count_test_patterns does not find any of the
test_pattern.

This patch calls use_test_buffer in order to force the compiler to really copy
the pattern to buf.

ChangeLog:

	* string/tst-xbzero-opt.c (use_test_buffer): New function.
	(prepare_test_buffer): Call use_test_buffer as compiler barrier.

1 files changed, 5 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index d88a7987f2..3e25bd7a18 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2018-07-26  Stefan Liebler  <stli@linux.ibm.com>
+
+	* string/tst-xbzero-opt.c (use_test_buffer): New function.
+	(prepare_test_buffer): Call use_test_buffer as compiler barrier.
+
 2018-07-26  Florian Weimer  <fweimer@redhat.com>
 
 	* htl/lockfile.c (flockfile, funlockfile, ftrylockfile): Use weak