RISC-V: Support more cores

I recently tried to run my RISC-V operating system with 16 cores, but I got an error that QEMU only supports up to 8 cores.

Then I researched a bit, and found this, which shows that the reason for the limit was Berkley Bootloader. However, BBL is now really outdated and replaced by new and better loaders, so it would be nice if the core limit could be increased again.