Genericise deletion actions that occur after empty_slot

This patch adds a generic "post_cap_deletion" step that is called by
finalise_slot. Previous to this, the only caps which had actions
required at this stage were IRQHandlerCaps -- it was required that the
IRQ bitmap be updated after the cap itself was removed (as the
invariants state that for any existing IRQHandlerCap, the corresponding
bit in the IRQ bitmap must be set).

By genericising this, we add the capacity for new, arch-specific post
cap deletion actions to occur in the future.

arm ainvs: wp rules for simple_ko setter/getter

Removes all trailing whitespaces

arch_split: invariants: tidied

arch_split: IpcCancel_AI [VER-567]

arch_split: skeleton arch files for AInvs