KVM: arm64: selftest: vgic-v3: Add basic GICv3 sysreg userspace access test

KVM: arm64: selftest: vgic-v3: Add basic GICv3 sysreg userspace access test

We have a lot of more or less useful vgic tests, but none of them
tracks the availability of GICv3 system registers, which is a bit
annoying.

Add one such test, which covers both EL1 and EL2 registers.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
Reviewed-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20250718111154.104029-5-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>