Draft: Fix #187: Disassemble imul correctly
When the immediate for imul doesn't fit in a (signed) 8-bit byte, we were printing the values an unsigned value. Tell the printer we have a signed value so that it prints out a signed number.
When the immediate for imul doesn't fit in a (signed) 8-bit byte, we were printing the values an unsigned value. Tell the printer we have a signed value so that it prints out a signed number.