[AVR] Implement disassembly of 32-bit instructions

This needed two fixes:

  * 32-bit instructions were read in the wrong order. The machine code
    swaps the two 16-bit instruction words, which wasn't undone when
    decoding instructions.
  * Jump and call instructions don't encode the lowest address bit,
    which is always zero. Therefore, the address needed to be shifted by
    one to fix that.

Differential Revision: https://reviews.llvm.org/D81961