KVM: arm64: vgic-v3: Reinterpret user ISPENDR writes as I{C,S}PENDR

User writes to ISPENDR for GICv3 are treated specially, as zeroes
actually clear the pending state for interrupts (unlike HW). Reimplement
it using the ISPENDR and ICPENDR user accessors.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231219065855.1019608-4-oliver.upton@linux.dev