riscv: build crash_mem ranges from memblock instead of resource tree

RISC-V crash 内存范围改用 for_each_mem_range 遍历 memblock 而非资源树，并让 CRASH_DUMP 选择 ARCH_KEEP_MEMBLOCK，为后续用 MEMBLOCK_NODUMP 过滤 vmcore 保留区做准备。