cmd/asm: Create empty RISCV arch
cmd/asm: Create empty RISCV arch

It can't actually assemble anything, but the tool recognizes it.

Change-Id: Ic325923dfe81f1fb53678019f1f4f69e9a18ae04

mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go