ZX Spectrum Next CPU (FPGA) syntax and code generation

Support the custom-designed Z80 compatible FPGA-based CPU that is implemented in the ZX Spectrum Next computer. This CPU implementation is 100% compatible with Zilog Z80 instruction set, adding a subset of new custom-designed instructions that increases versatility of the original Z80 instruction set and special support for the ZX Spectrum Next graphics hardware.

The instruction mnemonic syntax and opcodes are listed in the following link: https://wiki.specnext.dev/Extended_Z80_instruction_set

Mpm enables the CPU target with the -mzxn command line option that parses the additional instruction set for code generation. A -DZXN symbol is also automatically created for conditional assembly purposes.