riscv: Kconfig, asm/multikernel.h (hartid maps) and arch stubs #22

Part of the RISC-V port (see tracking issue). Depends on #8 (generic core cleanups).

• arch/riscv/Kconfig: select ARCH_SUPPORTS_MULTIKERNEL (depends on RISCV_SBI, KEXEC_FILE, HOTPLUG_CPU, !RISCV_M_MODE).
• arch/riscv/include/asm/multikernel.h: arch_cpu_physical_id() = cpuid_to_hartid_map(cpu), arch_cpu_from_physical_id() = riscv_hartid_to_cpuid(hartid). Hartids are sparse and may exceed NR_CPUS; never index arrays by hartid (the spinwait code does, cpu_ops_spinwait.c:26, and that is why it is not a model).
• MK_CTRL_BLOCK_SIZE: spawn context record + DTB + the fence.i entry stub page. No page tables, no trampoline pages.
• arch/riscv/multikernel/Makefile + spawn.c with stubs for every function in the "Architecture interface" section of include/linux/multikernel.h.

Goal: an rv64 build with CONFIG_MULTIKERNEL=y that links. Functionality follows.