Fix malloc tests build with GCC 10.

Fix malloc tests build with GCC 10.

GCC mainline has recently added warn_unused_result attributes to some
malloc-like built-in functions, where glibc previously had them in its
headers only for __USE_FORTIFY_LEVEL > 0.  This results in those
attributes being newly in effect for building the glibc testsuite, so
resulting in new warnings that break the build where tests
deliberately call such functions and ignore the result.  Thus patch
duly adds calls to DIAG_* macros around those calls to disable the
warning.

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

	* malloc/tst-calloc.c: Include <libc-diag.h>.
	(null_test): Ignore -Wunused-result around calls to calloc.
	* malloc/tst-mallocfork.c: Include <libc-diag.h>.
	(do_test): Ignore -Wunused-result around call to malloc.