kselftest/arm4: Provide local defines for AT_HWCAP3

Some build environments for the selftests are not picking up the newly
added AT_HWCAP3 when using the libc headers, even with headers_install
(which we require already for the arm64 selftests).  As a quick fix add
local definitions of the constant to tools use it, while auxvec.h is
installed with some toolchains it needs some persuasion to get picked up.

Signed-off-by: Mark Brown <broonie@kernel.org>