opcodes: blackfin: fix decoding of dsp mult insns

opcodes: blackfin: fix decoding of dsp mult insns

When assigning to a register half, the mac0 part of the mult insn
was not decoding properly.  It would always show a full dreg instead
of the dreg low half.

Once we fix the disassembler, we have to update a few of the gas
tests as their previous expected output was incorrect.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>