Treat vrgather immediate as unsigned (#222)

A literal reading of the current text would imply it should be signed (as that is the default and vrgather's description says nothing to the contrary), but negative immediates are useless on this instruction, as they would (after sign extension) be larger than 2^31 and thus exceed VLMAX on any implementation and just splat a constant 0 into the destination.

FWIW, binutils currently treats it as unsigned.