KVM: arm64: Properly save/restore HCRX_EL2

KVM: arm64: Properly save/restore HCRX_EL2

Rather than restoring HCRX_EL2 to a fixed value on vcpu exit,
perform a full save/restore of the register, ensuring that
we don't lose bits that would have been set at some point in
the host kernel lifetime, such as the GCSEn bit.

Fixes: ff5181d8a2a82 ("arm64/gcs: Provide basic EL2 setup to allow GCS usage at EL0 and EL1")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250430105916.3815157-2-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>