Fix string/tester.c for GCC 7 -Wstringop-overflow=.

GCC 7 has a -Wstringop-overflow= warning that includes warning for
strncat with a size specified that is larger than the size of the
buffer (which is dubious usage, but valid at runtime if in fact there
isn't an overflow with the particular buffer contents present).
string/tester.c tests such cases; this patch arranges for this warning
to be ignored around relevant strncat calls.

Tested compilation for aarch64 (GCC mainline) with
build-many-glibcs.py; did execution testing for x86_64 (GCC 5).

	* string/tester.c (test_strncat): Disable -Wstringop-overflow=
	around tests of strncat with large sizes.