RISC-V: make .insn actually work for 64-bit insns

RISC-V: make .insn actually work for 64-bit insns

Presently in this case, due to an undefined behavior shift, at least
with x86 cross builds I'm observing:

Error: value conflicts with instruction length `8,0x0000003f'

Eliminate the UB and extend the respective testcase.