Fix tst-cmp.c build with GCC mainline.

Building the testsuite with GCC mainline fails with
-Wstringop-overflow= errors in string/tst-cmp.c.  These are for calls
to strncmp and strncasecmp with SIZE_MAX size argument.  The tests are
deliberately using this size that would be dubious in normal code, so
this patch disables the warning for the calls in question.

Tested with build-many-glibcs.py for aarch64-linux-gnu.

	* string/tst-cmp.c: Include <libc-diag.h>.
	(strncmp_max): Disable -Wstringop-overflow= around call to
	strncmp.
	(strncasecmp_max): Disable -Wstringop-overflow= around call to
	strncasecmp.