arm64: cpufeature: Add GCS to cpucap_is_possible()

Since system_supports_gcs() ends up referring to cpucap_is_possible(),
teach the latter about GCS for consistency with similar features.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/416c7369fcdce4ebb2a8f12daae234507be27e38.1733406275.git.robin.murphy@arm.com


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