The makefile can install the riscv32-gnu-toolchain in /opt/riscv and qemu-system-riscv32 and qemu-riscv32.
Step 2 will make the vmlinux, I have modified it and it can run and get vmlinux 32bit.
But Berkeley now doesn't support 32bit-linux much, so I just can run qemu-riscv32 on user mode.

Like RISCV32-B group，if want to boot a RISCV linux kernel, only the RISCV64 linux kernel can run successfully now.

See it in https://github.com/riscv/riscv-qemu/blob/fe43ef40b2632891da9cb31c786ebea7dfe86394/README.md, Full-System Simulation using the Spike board and Full-System Simulation compatible with the SiFive U500 SDK, and this tow methods both can only support the RISCV64 linux kernel to run successfully.
So you can follow it to finish RISCV64 using the Spike board or the SiFive U500 SDK.