riscv: spinwait / M-mode fallback park page (optional)

[congwang-mk]

Part of the RISC-V port. Only for platforms without SBI HSM (CONFIG_RISCV_BOOT_SPINWAIT) or M-mode Linux (CONFIG_RISCV_M_MODE). Optional; do not build the common path around it.

cpu_ops_spinwait.c spins secondaries inside the kernel image on __cpu_spinwait_stack_pointer[hartid] (indexed by hartid, hartid < NR_CPUS required). That is the x86 hazard: a hart executing an image about to be overwritten.

• Host-owned MMU-off spin page: satp = 0, poll a per-hart mailbox, wfi, on match fence.i then jr entry with a0 = hartid, a1 = dtb. Twin of the arm64 spin-table item arm64: spin-table fallback park loop for platforms without PSCI #19.
• Host parks a hart by jumping to the spin page with satp = 0 (same sequence riscv_kexec_relocate uses: set stvec to the physical continuation, write satp).
• Spawn secondaries via stock spinwait pointing at the host-owned page is not possible (the pointers live in the spawn image); either keep spinwait harts inside the spawn and accept the hazard on halt, or give the spawn a mailbox-based cpu_ops. Decide when a real target appears.