Disable strncat test array-bounds warnings for GCC 8.

Some strncat tests fail to build with GCC 8 because of -Warray-bounds
warnings.  These tests are deliberately test over-large size arguments
passed to strncat, and already disable -Wstringop-overflow warnings,
but now the warnings for these tests come under -Warray-bounds so that
option needs disabling for them as well, which this patch does (with
an update on the comments; the DIAG_IGNORE_NEEDS_COMMENT call for
-Warray-bounds doesn't need to be conditional itself, because that
option is supported by all versions of GCC that can build glibc).

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

	* string/tester.c (test_strncat): Also disable -Warray-bounds
	warnings for two tests.