add riscv support

[Kreijstal]

It doesn't compile under riscv :(

user@starfive:~/proot$ make -C src loader.elf loader-m32.elf build.h
make: Entering directory '/home/user/proot/src'
.//arch.h:55:10: error: #error "Unsupported architecture"
   55 | #        error "Unsupported architecture"
      |          ^~~~~