S390: Fix FAIL in test string/tst-xbzero-opt [BZ #21006]

On s390x this test failed with:
FAIL: explicit clear/test: expected 0 got 1

In setup_explicit_clear, the buffer is filled with the test_pattern.
On s390x the memcpy in prepare_test_buffer is done by loading
r4 / r5 with the test_pattern and using store multiple instruction
to store r4 / r5 to buf.
If explicit_bzero is resolved in setup_explicit_clear, r4 / r5 is
stored to stack by _dl_runtime_resolve and the call to memmem in
count_test_patterns finds a hit of the test_pattern on the stack.

This patch resolves all symbols at program startup by linking with
-z now.  This omits the call of _dl_runtime_resolve within
setup_explicit_clear and the test passes.

ChangeLog:

	[BZ #21006]
	* string/Makefile (LDFLAGS-tst-xbzero-opt): New variable.