RISC-V: don't disassemble unrecognized insns as .byte

RISC-V: don't disassemble unrecognized insns as .byte

Insn width granularity being 16 bits, producing byte granular output
isn't very useful. With there being a way to specific otherwise
unknown insns to the assembler, use that same representation (to be
precise: its <length>,<encoding> flavor) for disassembly.