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

[Stefan Liebler]

Fix string/tst-xbzero-opt is 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 introduces a compiler barrier just after filling the buffer.

If okay, shall this be committed before or after the release?

Bye
Stefan

---

ChangeLog:

	* string/tst-xbzero-opt.c (prepare_test_buffer):
	Add compiler barrier.
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