kselftest/arm4: Provide local defines for AT_HWCAP3

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>
Link: https://lore.kernel.org/r/20250715-arm64-selftest-bodge-hwcap3-v1-1-541b54bc43bb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>