Fix failing test malloc/tst-interpose-nothread with GCC 7.

Fix failing test malloc/tst-interpose-nothread with GCC 7.

The test malloc/tst-interpose-nothread fails on s390x if built
with GCC 7 and glibc commit "Remove the str(n)dup inlines
from string/bits/string2.h. Although inlining"
(ae65d4f3c3995279ca458c460ebf8bab1885fa03) with output:
error: free: 0x3fffdffa010: invalid allocation index: 0 (not less than 0)

The destructor check_for_allocations in malloc/tst-interpose-aux.c is
called twice.  One time after the test-child-process has finished successfully
and once after the test-parent-process finishes.
During the latter invocation, allocation_index == 0.  GCC 7 is now inlining the
free function and calls unconditionally fail in get_header as
header->allocation_index (type == size_t) is always >= allocation_index (= 0).
Before the mentioned commit above, strdup was replaced by strlen, malloc and
memcpy.  The malloc call was also inlined and allocation_index was set to one.

This patch moves the already existing compiler barrier before the invocation
of free.

ChangeLog:

	* malloc/tst-interpose-aux.c (check_for_allocations):
	Move compiler barrier before free.