Disable -Wrestrict for two nptl/tst-attr3.c tests.

nptl/tst-attr3 fails to build with GCC mainline because of
(deliberate) aliasing between the second (attributes) and fourth
(argument to thread start routine) arguments to pthread_create.

Although both those arguments are restrict-qualified in POSIX,
pthread_create does not actually dereference its fourth argument; it's
an opaque pointer passed to the thread start routine.  Thus, the
aliasing is actually valid in this case, and it's deliberate in the
test.  So this patch makes the test disable -Wrestrict for the two
pthread_create calls in question.  (-Wrestrict was added in GCC 7,
hence the __GNUC_PREREQ conditions, but the particular warning in
question is new in GCC 8.)

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

	* nptl/tst-attr3.c: Include <libc-diag.h>.
	(do_test) [__GNUC_PREREQ (7, 0)]: Ignore -Wrestrict for two tests.

(cherry picked from commit 40c4162d)