kselftest/arm64: Add GCS as a detected feature in the signal tests

In preparation for testing GCS related signal handling add it as a feature
we check for in the signal handling support code.

Reviewed-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-v13-30-222b78d87eee@kernel.org


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