kselftest/arm64: Add very basic GCS test program

This test program just covers the basic GCS ABI, covering aspects of the
ABI as standalone features without attempting to integrate things.

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-v13-34-222b78d87eee@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>