arm64: Kconfig, asm/multikernel.h and arch stubs #9

Part of the arm64 port.

ARCH_SUPPORTS_MULTIKERNEL is def_bool X86_64 in arch/x86/Kconfig. Add the arm64 side:

• arch/arm64/Kconfig: select ARCH_SUPPORTS_MULTIKERNEL (depends on ARM_PSCI_FW, KEXEC_FILE).
• arch/arm64/include/asm/multikernel.h: arch_cpu_physical_id() = cpu_logical_map(cpu) (MPIDR, already u64), arch_cpu_from_physical_id() = get_logical_index(mpidr), MK_CTRL_BLOCK_SIZE (spawn context + DTB; no page tables, no trampoline pages).
• arch/arm64/multikernel/Makefile + empty spawn.c with stubs for every function in the arch interface (include/linux/multikernel.h, "Architecture interface" section) so the tree links with CONFIG_MULTIKERNEL=y on arm64.

Goal of this issue is only a linking arm64 build with CONFIG_MULTIKERNEL=y; functionality comes in the follow-ups.