#968 HuC6280 assembler support

[Philipp Klaus Krause]

The HuC6280 is a variant of the 65C02 with a few additional instructions. 6 of these additional instructions looks like would be obviously useful in SDCC code generation; also there seems to be interest in the TurboGrafx-16 community to use SDCC to target the HuC6280 present in the sound chip of the TurboGrafx-16. So we might want to add a huc6280 port based on the existing mos65c02 port. To do so, we need assembler support for the HuC6280.