ANDROID: KVM: Define mem_relinquish interface for releasing memory to a hypervisor.

On PKVM/ARM64 this uses the ARM SMCCC relinquish hypercall when available.

Memory relinquish interface is used by both memory ballooning and
by page reporting. It must be built if either is specified.

Bug: 357781595
Change-Id: Ifa85b641a48f348a2364cf8c6b06b6417f1eeedb
Signed-off-by: Keir Fraser <keirf@google.com>