opcodes/mips: disassemble unknown micromips instructions as two shorts

Before commit:

  commit 2438b771
  Date:   Wed Nov 2 15:53:43 2022 +0000

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

unknown 32-bit microMIPS instructions were disassembled as a raw
32-bit number with no '.word' directive.  The above commit changed
this and added a '.word' directive before the 32-bit number.

It was pointed out on the mailing list, that for microMIPS it would be
better to display such 32-bit instructions using a '.short' directive
followed by two 16-bit values.

This commit updates the mips disassembler to do this, and adds a new
test that validates this output.