kselftest/arm64: Add a GCS test program built with the system libc

kselftest/arm64: Add a GCS test program built with the system libc

There are things like threads which nolibc struggles with which we want
to add coverage for, and the ABI allows us to test most of these even if
libc itself does not understand GCS so add a test application built
using the system libc.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Tested-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-35-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>