Disable -Wformat-overflow= warnings for some printf tests.

Recent GCC -Wformat-overflow= changes result in some printf tests
failing to build, because those tests are deliberately testing the
handling of formats writing more than INT_MAX characters and the
handling of NULL arguments to the %s format, which GCC now warns
about.  This patch duly disables -Wformat-overflow= for the relevant
calls to printf functions.

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

	* stdio-common/bug22.c: Include <libc-diag.h>.
	(do_test): Disable -Wformat-overflow= warnings around fprintf
	calls outputting more than INT_MAX characters.
	* stdio-common/tst-printf.c: Disable -Wformat-overflow= warnings
	around printf call with NULL %s argument.