Ignore -Wrestrict for one strncat test.

With current GCC mainline, one strncat test involving a size close to
SIZE_MAX results in a -Wrestrict warning that that buffer size would
imply that the two buffers must overlap.  This patch fixes the build
by adding disabling of -Wrestrict (for GCC versions supporting that
option) to the already-present disabling of -Wstringop-overflow= and
-Warray-bounds for this test.

Tested with build-many-glibcs.py that this restores the testsuite
build with GCC mainline for aarch64-linux-gnu.

	* string/tester.c (test_strncat) [__GNUC_PREREQ (7, 0)]: Also
	ignore -Wrestrict for one test.

(cherry picked from commit 35ebb6b0)