opcodes/mips: use .word/.short for undefined instructions

While working on disassembler styling for MIPS, I noticed that
undefined instructions are printed by the disassembler as raw number
with no assembler directive prefix (e.g. without .word or .short).

I think adding something like .word, or .short, helps to make it
clearer the size of the value that is being displayed, and is inline
with what many of the other libopcode disassemblers do.

In this commit I've added the .word and .short directives, and updated
all the tests that I spotted that failed as a result.